It is pulled from his DeviantArt favorites, his VeryGoods likes, his Flickr favorites, his Instagram Favorites, his Dribbble likes, and some images from Twitter. The server then provides a random sampling from them as you scroll.
He originally built this as a way to find inspiration. Nat has found over the years that many people have created great art on dA that is interesting, beautiful, and/or inspiring. By taking a random section of these images, he hopes to find a way to break through things like creative block, and the like.
If you like this site, you might like walls.natwelch.com, Nat's site for storing all of the wallpapers he uses.
See the source.