I'm not really anal about the rules (though some art-thieves I've confronted in the past will probably disagree on that
), but I have two main rules: Keep your paws of personal artwork, and
credit any resources you use. (With personal artwork I mean individual pieces of art, as opposed to screenshots and manga pages and stuff like that).
I guess the thing that frustrates me most is that people clearly don't read descriptions. Aside from the using art without permission and bad crediting, he does say (in his later deviations) that he only edits. But from the comments it's obvious that 80% still thinks that he made it all, an he doesn't set them straight in his reply.