Since none of the images used are stored on the website's server, the artists could just reupload hardcore pornography on their sites and give them the same file name as the art taken. That said, if you're an artist concerned about someone (or a website) infringing upon your material, be careful to read the Terms of Service for websites you post on. Some, like Facebook, include clauses giving them a transferable license to distribute and use your material as they see fit if you post material. 

Otherwise, artists can issue DMCA Takedown Notices to servers that unknowingly host (e.g. distribute) their material (as I see there are a lot of artists in this thread).