i would consult thier host.... hosts get really pissed when people steal copyrighted images... or trademarked images.. or whatever... if its on a web site... the host would either damand removal, or completely shut down thier site.
It's happened allot to me, the best you can do is contact whatever site is hosting them and tell them there's been an intellectual copyright infringement. That's one of the reasons I seriously shy away from using anything I haven't created myself.
actually, i wouldnt mind it so much. i mean, non of my art is really that good yet...decent enough maybe, but i still wouldnt really mind. thats just me though, art is a hobby, not my life. in other words, im not going to get upset about anything...not to mention i normally do art for other people and not myself anyway.