it is on the head of the copyright holder to enforce their copyright at least in us legal system. The artists if they are serious should start suing people then for the revenue they think they will have lost from a person using their picture. So start suing people and let the courts...
if anyone desires that they can ask, they can hit reply and take the link to the work, if they are to shy to say hey this is cool who did it then they didn't really have any interest in purchasing anything.
i doubt many people from an RP are going to go looking for an artist that may or may not do commissions. If they wanted to get commissioned art they would be looking for it already. If a person does want to hunt the artist down its one reverse search away or even a reply away to go to...
I really dislike when real photos are required and art is not allowed. Like I get not wanting super anime pictures but there is a gradient. Photos just feel so odd like one day they might search their photos and read about using them in some roleplay.