Okay first off, yes stealing is wrong. you shouldn't do it. and really it's pointless because, it doesn't make you a better artist or change the fact that the other person has skill. but then again, people shouldn't go out looking for art thiefs. all these art thieft "hunters" are really annoying me. all they are is butthurt trolls. if you find stolen work, get the whole story because it may not be stolen at all. and if it is, simply denunciar them and go on. don't even tell them you did. it will just cause drama. and im sure none of us want that. and for the record, yes usernames, signitures, and avatars/icons are copy righted. they are directly about the person owning them. yes, if they use a public image you may use it and user names can be simular but if you are directing stuff to a spesific person then it's copyrighted. expecially on deviantArt. and they inforce it more. you can be banned and even turned in to the police.
