How to Change the GDM 2 Background

In the past, Gnome GDM's appearance could be changed with themes. This was changed when GDM was rewritten for Gnome 2. GDM now uses gconf and the keys are documented in the Gnome Library. At first I was having little luck with these as I was naively running gconftool-2 as root. You have to run the gconftool-2 as gdm:

sudo -u gdm gconftool-2 -t str -s /desktop/gnome/background/picture_filename /full/path/to/your/background.png