You're inserting morals into something that's hugely insignificant. If you find some art online, then just use it. It's just a roleplay after all, and there are millions of pictures, arts, etc already readily available online and trying to curtail people using someone else's artwork in this manner is pointless and a waste of everyone's time. Besides, artwork should only be used as supplemental material to enhance your writing, and because of this, you might as well consider using their artwork for this as a big whoopdeedo.