Now you've found the photo and the infringer, you have a decision. Do you want to go for cash, or do you want it removed from the web (not least so that others cannot nick it)?
Trying for some cash
In UK law, you are entitled only to claim the actual financial loss you have suffered as a result of an enfringement of your copyright. If you were to go to court, however, defending the case would cost the infringer lots of money. So it would be reasonable to invoice them for twice what you would have charged, had they asked nicely.
Of course, going to court would cost you money. Small Claims Courts in England and Wales do not currently take cases involving copyright.
(In Scotland, the Small Claims track of Sheriff's Courts may take such cases, and have indeed been known to award sensible damages for flagrant infringement.)
The NUJ proposed in 2006 that in the interests of justice there must be a Small Claims procedure that deals with copyright matters. In January 2010 Lord Justice Jackson proposed the same in a government-sponsored review of civil litigation. The NUJ is pressing for this to be implemented promptly.
It is (also of course) even harder if the infringer is outside the UK. The NUJ proposed, and the International Federation of Journalists accepted, that journalists' unions should support each others' members in such claims. NUJ members should contact the Freelance Office to ask whether a journalists' union in the country in question can yet help.