Deviant Art is a FANTASTIC site for artist off all sorts. They have one of the best sites for posting/displaying fan generated Avatar Artwork. Check them out. You won't be disappointed! (warning! BSG.com is not affiliated in any way with Deviant Art. Though they have maturity content filters in place, some of the artwork may contain more teen related/adult related content. Also, some of the artwork may contain spoilers to some of the episodes)
Avatar the Last Airbender on Deviant Art
For Specific Charcter Art, click on the links below:
Minor Avatar Characters on Deviant Art
Photobucket is another image hosting service which BSG.com often uses to uploads pic onto our main forum. Here you can find both screen cap stills, Avatar artwork and gifs. BSG.com is in no way affiliated with Photobucket. Though I have previewed the content, there may be images which are teen/adult related in content and there are no maturity filters currrently set in place on this site.