Where civil rights are being infringed upon, ie. IP/Copyrights etc. you might simply send a shout to www.ACLU.com & www.BBB.com to see what sort of active remedies might be able to be enacted through the civil court system? Say, with a Cease & Desist order? At least until steps can be taken if further infringing activity continues, for reimbursement of previously stolen art - and recovered or erased from digital media on premises, and from online account storage mediums.
Just a thought anyway, many people don't go that far - but if it's your IP/rights that are being infringed then you should do more than just warn your friends!
Unfortunately, it is usually on the infringed person to take remedial steps in reporting the issue, as there hasn't been many watchdog systems in place! But with new SOPA related copyright laws and legislature continually being developed there may soon be a common avenue of recourse for copyright holders. Whether it be personal efforts, or through government authorities.
Aside from all the legalese mumbo jumbo, I'm a share-alike content creator myself! But if I saw a piece of my work being sold and not shared as it was intended, then I would definitely take steps to stop it's sale! And insist that it be distributed freely!
I'm no expert - but I would think that would be up to their legal dept. and how much fine print was done for pictorial/fan art resources based on the franchise in question.
My advice would be to ASKbefore spending time on art that may or may not be legal to display without knowing before hand or having proper model release permissions to do so! To avoid paperwork that isn't art!
Now, if it had been a picture of a painting you did - I believe that would fall under a different category all together!