Many, many things there are stolen from Interfacelift or DeviantArt. (Aka, someone other than the artist posts them there, without the artist's permission. Often they are downscaled, cropped, or have the artist's signature removed.)
OP, those two sites have enough wallpapers to last you years. A tip for finding wallpapers at dA, if you didn't already figure it out: While they have an actual "wallpapers" section, a lot of the best stuff isn't there. Most people file their stuff by what it contains, even if they sized it specifically for wallpaper use. (Plus those sizes happen to look best blown up in a full screen slideshow too)