If it happens on this site, it is your responsibility as an Administrator to investigate it Ven, even if it doesn't have to do directly with the game, because in the off chance that an artist did decide to take a user to court over the matter (and I've seen it happen before where an artist sues someone over matters of art theft), you could be seen as liable because you didn't do anything about it.
If someone reports art theft and has proof of it, remove the stolen material. It's as simple as that.