How do I change the desktop background from command line?

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?

It's a gconf setting: /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>

Obviously, replace <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:

gconf-editor /desktop/gnome/background/