Walls by JFL - Widescreen (Free)
Wallpaper Zone
GNOME-Look.org
Vladstudio.com – free wallpapers
HQ Free Wallpapers
Computer wallpapers / wallbase.cc
This App Sets the Bing Image of the Day as the Wallpaper on Linux - OMG! Ubuntu!
(Xubuntu) How to set the wallpaper using the command line? - Ask Ubuntu