I recently stumbled with this great French site, which contains some of the most amazing Linux wallpapers I have found. Many Linux distributions are featured, including the usual suspects, like Ubuntu, Linux Mint and Fedora, but also some other distros that are perhaps not as popular yet, such as CrunchBang or Pardus.
I personally found some of the Ubuntu backgrounds particularly good looking, but there is interesting stuff for everyone, really. Another great thing is that the wallpapers uploaded are ridiculously good in quality. Picture resolution is usually off the charts, so even huge or even dual screen setup users should find something interesting.
Now, stop reading and go get those amazing LINUX WALLPAPERS!