I upgraded to natty and the old background images are gone. How do I get them back?
The wallpapers, or backgrounds, for maverick are listed here:
You can download the jpg's one at a time. If you want to download them all at once you can go to this page and download the ubuntu-wallpapers-*-tar.gz file
Unzip the file (gzip -d ubuntu-wall*gz), and untar it (tar xf ubuntu-wall*gz). The images will all be in a directory.
Get the image you want by either method.
Then right-click on your desktop, choose "Change Desktop Background", then click "Add" and choose the image you want as your background. It should now be your new background.
You can get old (and current) stock wallpapers here. Download the deb matching the release you want, but save them, don't open them in software center. After downloading them, right-click the file and pick Open with Archive Manager. Browse the package and extract the wallpapers you want.