Aside from our own Art & Design section (make sure you give credit if you're borrowing from our artists!), both DeviantArt and Pixiv are great resources for Pokemon art. The latter is in Japanese though, so while there's lots of great stuff there, it might be a little difficult to navigate.
There are tons of great Pokemon wallpapers on Zerochan - it's where I get basically all of my anime-related wallpapers. Some of the less complex ones that you'll find are great for avatars. Hope you find something that you like!