They are choosing to share their work in a very public place (imgur, tumbler, facebook, you get it) and shouldn't be surprised if said image is snagged by a roleplayer or whoever for personal use. I realize that doesn't help you, but it does make me wonder what is so harmful about having an image up on here. Do they not like the exposure? Would they be happier if we linked back to their blog or whatever? this "problem" would be easier to deal with if we knew exactly why they had a problem with their image appearing on a random RPG board.

It might also be worth investigating if these people even have a leg to stand on as far as their copy write claims go. From my understanding, most image boards basically have their own rights that they apply to an image when you decide to post your images to it. 

So yea, I think first I'd like to know why artists find their images winding up on an RPG forum is damaging to them.