Changing the background in GDM

Run this command as root:

gconftool-2 --direct --config-source=xml::/etc/gconf/gconf.xml.vendor --set /desktop/gnome/background/picture_filename --type=string "/path/to/picture"
gconftool-2 --direct --config-source=xml::/etc/gconf/gconf.xml.vendor --set /desktop/gnome/background/picture_options --type=string "stretched"

Sourced from http://lizards.opensuse.org/2010/07/25/how-to-change-gdm-backgroundwallpaper/