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.