I combined the script from webupd8 to get all the wallpapers from the 2009 (http://www.webupd8.org/2009/11/automatically-download-all-wallpapers.html)photography contest with the cosmos wallpaper set from Gnome 2.28.
It uses all the wallpapers downloaded from NatGeo webpage (170 wallpapers more or less) and makes a xml file to change automatically the wallpaper every 29 minutes.
I understand that there has been an attempt to introduce malware in a gnome-look hosted package, but this script is completely innofensive!
To use the script just download it and make it executable for root:
sudo chmod a+x NatGeoWalls
the execute it with sudo:
sudo sh NatGeoWalls
Wait a few minutes to download all the photos and create the xml, and finally change the wallpaper in the appereance settings windows