There are 2 basic scripts which don't do much, just download all the wallpapers from the National Geographic International Photography Contest Desktop Wallpaper for 2008 and 2009. That means almost 400 wallpapers.
For the 2008 script, you can edit the resolution. Simply change res="1024" in the script with res="1280" or res="800" (width). The 2009 script will only download 1600px width wallpapers (only these are available on the National Geographic website).
To use the script, simply place them in a folder, make them executable:
chmod +x natgeo2008.sh
chmod +x natgeo2009.sh
And then run them:
Don't worry if you get some "file not found" errors, those are because some wallpaper albums have less than 26 images and I have made the scripts try to fetch 26 images from each album (some have 26, some less).
The scrips are malware free. You can check them out - there are only a few lines of code.
The reason you may want to download so many wallpaper is to use them with an application such as Desktop Drapes.
See also: Lucast created a modified version of the 2009 script which creates an .xml file for use in Gnome 2.28 (like the Cosmos wallpapers): http://gnome-look.org/content/show.php/NatGeo+Wallpapers+script?content=117267 (see also some help setting it up: http://www.webupd8.org/2009/12/natgeowalls-creates-xml-wallpaper-set.html).
More info: http://www.webupd8.org/2009/11/automatically-download-all-wallpapers.html