[Linux (Gnome) only]: Lucast @ Gnome Look has created a script which uses my NatGeo wallpapers download script to get all the wallpapers for 2009 (~170 wallpapers) from the National Geographic website and then it creates a .xml file similar to the Cosmos wallpaper set in Gnome 2.28 which you can then use to automatically change your wallpaper every 29 minutes.

I have checked the script and is malware-free so it's safe to use!