It would make sense for art thievery to fall under the jurisdiction of plagiarism rules, since both are essentially forms of intellectual property appropriation. Without bringing the potential and complicated legal liabilities into it, I think there's a place to separate benign neglect from malice. A case of someone pulling the first result they're pleased with off google image results is one thing and mostly harmless (these people will usually comply with requests for attribution, etc.) is completely different from someone advertising work as their own when it clearly is not. The latter case is genuinely malicious and deserves to be dealt with harshly.

As for years-old content, two options exist: I see no harm in removing content from long-dead threads, however it raises an interesting question of how and why a person in question is finding the offending art in these threads. I won't assume malice without probable cause, but I find the pretense there not quite so black and white. Your other option is to deny removal requests on old content on the basis of it being dead and buried; a statute of limitations if you will. The biggest problem with denying removal requests on that ground is potential basis for sheltering copyright infringement and that's a sufficiently large issue in its own right.

So in regards to the first major query of having a policy or not, yes. That's a no-brainer that there should be some formal policy for something that's already come up three separate times. In regards to protections of artists vs members, copyright law somewhat binds you in that situation. You can only say no up until someone files a DMCA takedown request at which point your hands are tied or the site as a whole becomes liable. You can have a thorough process to weed out cases of the claimant not actually being the artist or otherwise acting purely out of malice, but at the end of the day if they want to be a dick about it, DMCA lets them do it.

In instances where the user on the site is responsive, the situation is most likely a lot easier ... or in a few rare cases much more problematic. Two cases exist: case one is where the artist in question has failed to reach out to the user in question prior to contacting staff. So long as the user is responsive, this is an easy case to solve with minimal mediation. The second case is problematic as the artist [i]has[/i] reached out but was met with refusal on the user's end. Here you already have two parties in heated disagreement with legal ramifications on the line. [i]Best of luck.[/i] My input is that most responsive users will be happily agreeable to attribution, using an artist's original source over a rehost, etc. They'll be less agreeable to an insistence on removal, but if you have an open line of communication, that's better than nothing.