In general, yes, they must, and if they use your copyright work without your agreement they commit an unlawful act.

There are a few exceptions:-

• If you have previously assigned copyright to someone else you have surrendered all ownership and negotiating rights. The copyright holder must grant permission.
•  Certain types of usage are regarded as 'fair use' and are allowed by copyright law. It is still polite for the user to ask, but they are entitled to use the work so long as it really does fulfil the criteria. Infringers frequently excuse themselves on the grounds of fair use meaning not-for-profit, or that any image found on the web is eligible, or similar misunderstanding of the law.
• If you have previously released the image under licence, usage within the terms of the licence does not require permission. Uses outside the scope of the licence are still protected by copyright law and require your agreement. For this reason do not grant licences that allow wider usage than necessary.

Copyright law permits some uses without the need to ask permission of the copyright holder, and these specific instances are called 'fair dealing' or 'fair use'.

Fair use of photographs comprises:

• Personal copying for the purpose of personal study or research.  This means it is OK to copy a photograph or a published photograph provided it is one copy made for the personal use of the person making the copy. Multiple copying, such as that a teacher might do for an entire class, is outside the scope of fair dealing. NB: US law differs significantly here, as virtually any educational use within schools or colleges is treated as fair use. US educational establishments will often be completely unapologetic about use of photos even in their marketing material, and even where UK photographers are concerned.
• Copying for the purpose of criticism or review is allowed as long as there is proper attribution. This allows for example, book reviews, or for someone to talk about your work on a web page. This fair use is widely misrepresented or misunderstood by webmasters, who seem to believe it means any attributed use is allowed. They are wrong : criticism or review is the key context here.

Images in the public domain lack copyright protection and may be freely used. This arises on expiry of copyright 70 years after the death of the author.

It is possible for photographers to effectively create public domain status for their work by granting a broad enough licence, eg the Creative Commons Attribution(BY) licence which permits any use on the condition a byline is provided..

It is important to realise that the issue of such a licence is effectively irrevocable. You cannot later revise the license to more restrictive terms, since existing copies will undermine any attempt to assert the subsequent terms.

There are three different methods and it is essential to use the correct and appropriate method in order to avoid unintended consequences.

Permission is a simple, informal agreement that the other party may make use of your work. Really, this means just saying 'yes' to their proposed use. You would be wise to specify exactly where and for how long and in what context they may use it, but if you start asking for payment or a byline in return you are creating a license instead. Informal permission stops short of forming an enforceable contract, so you have no real redress against misuse except copyright law, but conversely the other party can impose no conditions or penalties. Permission is really only appropriate for non-business use by friends and family.

A license is the usual form of agreement, and comprises a legally enforceable contract. A contract is an exchange of value between two parties, and imposes terms and conditions on both which specify the nature of the contract. It is essential that you read the terms offered by the other party and are careful about specifying your own. The small print of commercial publishers, organisations inviting reader submissions and competition rules must be read carefully as they often include copyright grabs and impose serious legal liabilities. Licenses are subject to contract law, and damages my be sought by either side for failure to complete the contract.

Assignment of copyright permanently and irrevocably surrenders all copyright and transfers it to another party. Assignments must be in writing, but are sometimes buried in contract terms and conditions or simple forms, so be very careful. Once you have assigned copyright you cannot use the photo you made without permission of the copyright holder.

First British Rights (also known as First British Serial Rights) is the traditional form of licence granted to commissioners of photography. It allows the client exclusive first use of the work produced, after which the photographer regains full rights to re-sell the material. If the commissioning client later wishes to make further use of the work by republishing or publishing in additional media or territories, a further fee must be negotiated.

The advantages of First British Rights are: 

• The client need pay for only the minimum rights that they need. This keeps their costs to a minimum.
• The possibility of resale allows the photographer to work cheaper than is viable for work that cannot be resold.
• It is fairer to both parties and recognises the intention of copyright law, that freelances are independent small businesses who need to retain the title to their assets.
• The gearing of use to reward is an incentive for the photographer to try hard to produce work of maximum value to the client
• Exclusivity is protected for the client. The photographer cannot sell the work to a competitor or anywhere else until after the client has used the photos. In the case of magazines and newspapers, resale rights are restricted whilst the edition remains on sale.

The disadvantages are:

• Keeping track of reuse across diverse media is practically impossible for the photographer, who must rely on client honesty
• The administrative burden of tracking further uses and negotiating additional usage fees is inconvenient to clients
• Clients may feel they are being 'held to ransom' over fees for subsequent use.

In recent years commissioning clients have often used market forces - the oversupply of hungry photographers - to move away from First British Rights contracting. It is now commonplace for commissioners to require copyright assignment - exactly the right that an employer has over an employee's work but with none of the costs - or a perpetual licence to republish without further fee. Either deprives the photographer of revenue, but fees have not increased to reflect this. Worse, it has led to a perception of photographers as interchangeable and disposable content suppliers rather than creative partners.

A rights-managed (RM) license is one where specific, defined reproduction rights are granted to the client, usually limited to a set period of time. A RM license will typically comprise a package of defined usage rights specifying size, media, purpose (eg editorial, promotional, advertising), and duration of the agreement.

The advantage to the photographer is that they retain precise control over how and where and for how long the image is used. If the client subsequently requires other uses they must be negotiated as additional license terms.

The disadvantage to the photographer is that RM licenses are necessarily modular and complicated to price. This complexity makes RM somewhat offputting to clients and especially hard to formulate automatically for online sales (although the PLUS! initiative has devised a pricing matrix to simplify this).

The advantage to the client is that they buy only the rights they need, keeping cost to a minimum. Further, exclusivity is possible and may be guaranteed via RM terms.

The disadvantage to the client is that further usage beyond the licensed terms will incur further negotiation and fees. Usage requires active monitoring and management by the client, who risks infringement by usage outside the terms. There is also a concern that they may be 'held to ransom' for license extensions, although this may be avoided by prior agreement of prices for possible expanded usage in the future.

A royalty free (RF) license grants a broad bundle of reproduction rights to a client. Typically this will include unlimited repeat use in any media, at any size and for any purpose, by the client.

The advantage to the photographer is the easy sell of a proposition that appears extremely generous, almost equivalent to copyright itself. RF licenses are perceived much as 'all you can eat for £10' bargains by clients. In fact the small print may impose conditions such as maximum size, or prohibit advertising use. RF licenses are also well suited to e-commerce and immediate download , in contrast to the complex modular pricing of RM licenses.

The disadvantage to the photographer is that RF turns photographs into commodity items. As with any commodity, volume and price are the key factors. All control of usage is permanently lost as soon as an image is issued with an RF license. The worst aspect of RF is that many clients have come to expect RF licenses even for commissioned work that has little or no resale value, yet with no compensating increase in fees. Further there is no ongoing relationship with the client after delivery.

The advantage to the client is that RF grants almost complete control over when, where and how many times they use the image and at a fixed price. As price is not geared to usage in any way  they do not need to monitor or manage use. 

The disadvantage to the client is that exclusivity is unobtainable. The basis of RF pricing is volume, and that means the same image may be purchased and used by competitors. RF stock is often sold as collections  at very low per-image prices. This looks irresistible but few clients ever use more than a small percentage of the rights bought. They may end up paying more than if they had licensed specific rights via rights managed (RM) licenses.

Creative Commons is a form of licensing developed by the Creative Commons Foundation, which aims to facilitate free sharing of creative works within limits specified by the license terms.

CC licenses are modular and the following components may be mixed and matched to achieve a variety of different terms:

• Attribution (BY): Licensees may copy, distribute and display
  the work and make derivative works based on it only if they give the
  author a credit as specified by the licence.
• Noncommercial l (NC): Licensees may copy, distribute and display the work and make derivative works based on it only for non-commercial purposes.
• No Derivative Works(ND): Licensees may copy, distribute and display  only exact copies of the work, not derivative works based on it.
• ShareAlike (SA): Licensees may distribute derivative works only under a license identical to the license that governs the original work. 

EG a BY-NC-ND licence would permit only bylined, non-commercial  copying but no derivative works. Omitting the byline or commercial use or making derivative works would break the terms of the license and would be actionable.

CC licenses are not without problems, both real and potential:

• Their complexity discourages people from reading or understanding exactly what the licence permits. CC is often assumed to mean much the same as free of copyright by image users, and that misunderstanding can encourage infringements.
• Despite CCF taking care to formulate terms that are in theory valid in many different countries there have been very few instances where the courts have tested their enforceability.
• Once an image has been released with a .'liberal' licence it is practically impossible to revert the terms to more restrictive permissions.
• Unforeseen outcomes may occur. For instance, if an image is released for non-commercial use and is then used by a non-profit site for purposes.that the author deplores yet finds his name alongside. As the license terms have been complied with there is nothing the photographer can do.

It is hard to see any benefit of CC licensing applied to photographs compared to traditional retention of full copyright. The latter does not preclude free usage or sharing, it just means the author's consent must be obtained,. The photographer therefore retains full control of how, by whom, where and for how much money.

A license is a contractual agreement that permits usage of copyright work in exchange for something. This can be payment of a fee, but may be anything of 'valuable consideration'. A byline or link to your site, or entry into a competition may be offered, and that is enough to form a legal contract if you accept it.

License contracts may be terribly simple, but with copyright they are often made very complex by one or both parties stipulating terms and conditions that define what will be agreed.

Never agree to contract terms unless you are able and willing to honour them. A license is a legally binding document.

A 'copyright grab' is a contract term that acquires an assignment of copyright or a licence of such wide scope that it removes most or all control of the image from the copyright holder.

Copyright grabs have become extremely widespread in recent years, and their usage by unscrupulous clients is so commonplace that you must carefully read the terms of any license proposed. They are often masked by reassuring statements that you retain copyright, but then go on to propose 'a perpetual, irrevocable, fully paid, royalty free, sub-licensable right to use, adapt and incorporate into derivative works the image for any purpose in any media including those yet to be invented, in all territories across the universe' or something similar.

Basically this means they can do absolutely anything they like with your photo, forever, and you have no further say in the matter whatsoever. And they get to keep all the income they generate too.

There is absolutely no sensible reason why anyone should ever agree to such terms. And if you do, you will almost always find an accompanying clause that places unlimited legal liability for costs onto you should their usage hit legal problems.

You might expect to be very handsomely paid in return for agreeing such generous terms, but copyright grabs often turn up in photographic competition rules and the small print of websites that invite users to send in material in return for nothing more than a byline. They are also a frequent addition to the terms of commission of newspapers, magazines and other clients, with little or no extra payment offered above that which they previously paid for First British Rights. All professional photographer organisations strongly oppose such exploitative terms.