With the advent of the DMCA, sites have protections from user postings ONLY if they remove items flagged by users who claim to be the copyright holder. You cant action things that are pointed out as stolen unless the person reporting it is the copyright holder. It is possible as an admin to remove items which you think are stolen BUT if you do that then you set the precedent that the site is then fully moderated for copyright accuracy so that then items that are not removed are deemed to have been chosen to have been left there by the moderators. Therefore, the general consensus is that you cant remove artworks which you think are stolen unless you have had it pointed out by the copyright holder that they are.
This is one of those weird aspects where the DMCA was supposed to have stopped people posting stolen copyright on forums but the reality is that it has prevented moderators from removing it.
So - if you are the copyright holder for the art then ask them to remove it, or find who the copyright holder is and tell them that they need to report it and usually sites will do so. If you claim to be the copyright holder and in actual fact you are not then the claimant is leaving themselves open to a claim of fraud instead of the hosting platform.
We have a FAQ on here about how to make DMCA claims on this site and we generally follow the usual process. Robbie is the final site owner and has access to the lawyers which is what part of the site subscriptions help fund - not that we have had to use their services very often as this is a pretty copyright abiding place given that we have a lot of content creators here as well as consumers of it.
Also, this site does not claim to own any of the art posted to it, nor does it take part of the commission for art sales, and its up to the requester and the comission artist to sort their own deal out and take their own reponsibility for it and as such the artist gets 100% of the comission and takes 100% of the risks for it too.