And they resized it badly so it looks fuzzy. Nice work.
Sadly, unless you registered it, all you can do is email them and tell them that it's a copyrighted image that they are using without permission and that you would like them to take it down or give you credit.
If they say they bought it, tell them you need to the a receipt (since now you have to chase down the person person selling it), and tell them you still want them to give you credit.
If they ignore all of that, you can either forget about it, or lawyer up, but you can't recover punitive damages, assuming you didn't register it. You can probably recover the legal fees, maybe. But all they can be forced to do is take it down (unless you get them to settle, of course, since laws don't really apply to settlements out of court).
jon