It's called FANART!!!!!
Most people who make wallpapers, avatars, banners, fanart etc. of their faves shows, movies, whatever, use screenshots, which is what I do! However, I customize the images so they are more interesting, pretty to put on your desktop, whatever!
I am not the only one who does this, so don't act like you don't know what fanart is!
DA has a category for it and another category called cutomization! If you don't like this kind of art join a new site that does ban it because DA doesn't! I looked in the rules and it doesn't say you can't submit recoloured images. All it says is that you cannot claim the original image as your own, which I don't!!