How can I change my desktop background from command line?
Edit: In later versions of Ubuntu (this question was written in 2011, after all), things moved to gsettings. See: How to change desktop background from command line in Unity?
/desktop/gnome/background/picture_filename. You can set its value by running the following:
gconftool-2 -t string -s /desktop/gnome/background/picture_filename <path>
<path> with a full path to the background image you want to display.
If you want to see what else is available from a nice GUI, run: