What you can do, if anything, depends on circumstances.

As with any infringer, the hope is that negotiation and commonsense or threats, will prevail, avoiding the costs and risks of legal action. However it's best to know what is possible so you can adopt a measured and realistic response.

The best situation is that the infringer has local offices and assets in the UK. If that is the situation, they are accessible to UK courts.

If the infringement is a web publication it can be useful to approach the ISP to deal with an evasive infringer. Overseas ISP's may have parents in UK, and one large infringement was dealt with by approaching the UK parent of an Indian hosting company, resulting in the site being removed from Indian servers.

If the infringer is using GoogleAds, you may be able to have their account cancelled, thereby depriving them of revenue. Google's IP policy provides a mechanism for doing this, although it is quite laborious. 

If the infringer is located in the USA and the work is registered with the US Copyright Office, you should not find it too difficult to locate a US attorney who will take your case through the US courts on a contingency fee basis. If it's an unregistered work, they won't be interested because it isn't likely to be financially rewarding to try and recover damages. You can however get a US attorney to apply for a take-down notice under the Digital Millenium Copyright Act (DMCA).

One thing to watch out for is that 'fair use' in USA extends to any educational use, and you will have no success against US schools and universities using your work for any purpose they wish.

There are also esoteric legal arguments surrounding what comprises 'commercial' use. In the USA editorial use is often distinguished from commercial use, because the press is protected as a constitutional freedom. In the last year or so this has given rise to an argument that press use is a constitutional obligation, therefore is not commercial and hence is fair use. This seems crazy because press publishing is commercial, done for profit, and they don't give away their product or copyright. The debate continues and is best ignored : until a test case has established otherwise they owe you the money.

Infringers in EC countries should become much easier to get at from January 2009, when the cross-border pursuit of damages will become available from any EC country rather than having to take action within the infringer's national jurisdiction. The process promises to be little more difficult than current Small Claims procedures within UK, with the facility to initiate online. If you suffer an EC infringement it seems likely to be worth waiting until next year to take action. We will report more on this as the details become clearer.

Elsewhere in the world you have little choice except to find a lawyer within the home country of the infringer and pay them to take legal action locally. This can bring you into contact with legal professionals and processes that will test your patience and wallet, so is best reserved for serious infringements where large sums of money are at stake.

In some cases local photographer or journalist trades unions may be able to advise or assist, and have done in Australia and Canada in particular. You probably want to start by asking the advice of your own TU or other professional organisation, if you have one. Similarly approaching the trade commission or bureau of embassies can assist. Some countries are sensitive about their trading reputation and will at least offer advice on how to resolve the situation. Others will sound sympathetic (or not) and show you the door.

Being realistic is essential. In many instances, your loss of income will be far exceeded by the costs of attempted recovery, and even if you win a judgement, enforcing it against people who have few or no assets and are thousands of miles away. Trying to get your £15 from 'What Yurt?' in Ulan Bator will simply not be worthwhile.

Having said that, it is essential to at least try and get the offending material removed and ensure that the infringer knows they owe you money. If you simply ignore infringement, you potentially weaken any future case for damages within the UK against some other infringer. If your failure to assert your copyright comes to light the argument could be made that you are simply now taking advantage of the court to demand money for images you had previously allowed to be used for free.