I was in a situation like this about a year ago. A car broker site (like autotrader, but much smaller) was using photographs I'd shot of cars as "stock" images for the site. I wrote them a letter pointing it out and saying that they could purchase the right to use the images if they wanted, or just remove them immediately. They took down the photos.

If you want to sue, as has been suggested, you have to realize that you will only ever be awarded damages that you can prove. If you go to court and say "Uhm, I think I could have sold the rights to these photos for ten million dollars" the court will want to see that you have previously sold photos for that amount, or that it is the industry standard price for this kind of use.

Are you a professional photographer? Were you really damaged by their use of the photos? If the answers are no and no, then your best bet is to write a letter offering to discuss how they're going to compensate you for future use of the images. Otherwise, they'll have to take them off the site. Chances are they're going to refuse to pay and just take the images down.