Hi there, welcome to Newgrounds!
The art and audio portals use a system called scouting. In essence it means that established artists are on the lookout for new artists, and if they deem their work high quality enough, they can scout them so their work appears on the respective portals. This is a way to have some quality control on there.
If I recall correctly, for the art portal you need at least 3 piece of art before you can be scouted by someone. In any case, you can have a read here for more information about scouting.
As for the thumbnail issue, I'm not sure what's going on there, perhaps your browser is caching the old thumbnail. Try pressing ctrl+R or cmd+R on your keyboard while on your user page and see if that makes it update. It forces the browser to reload all of the files associated with the page rather than relying on old versions it had cached.
EDIT: oops, looks like Mrccbirdguy beat me to it while I was writing my reply!