Desktop wallpaper changes only apparently
I had problems with my desktop wallpaper after uninstalling Kuvva as well. In my case, removing the desktop plist file solved the problem.

To do this, run the following command from the terminal:

rm ~/Library/Preferences/com.apple.Desktop.plist


Then log out and log back in. Be aware that this will also return any other desktop settings to their defaults. If you're uneasy about just outright deleting this plist, you can always make a backup copy first.