To change the gdm login screen background without a gdm-setup, which still isn't ready for Fedora9's 1.4.99 Xorg server, change the waves.xml file in /usr/share/backgrounds to reference the image you wish to use as the login screen's background.

You can remove the time references that change the background according to time-of-day, or use them and change your background accordingly. XML is a simple markup language similar to ASCII just with XML tags. Edit the fields in between the <...> TEXT <...>


┌────[root@xps-m1710]───────[08:19:16]───────[/usr/share/backgrounds]─────
└──> ls -ltr
total 40
drwxr-xr-x 2 root root 4096 2008-06-19 20:43 waves
drwxr-xr-x 2 root root 4096 2008-06-19 20:44 translucent
drwxr-xr-x 2 root root 4096 2008-06-19 20:44 tiles
drwxr-xr-x 2 root root 4096 2008-06-19 20:44 nature
drwxr-xr-x 3 root root 4096 2008-06-19 22:26 images

┌────[root@xps-m1710]───────[08:19:17]───────[/usr/share/backgrounds]────
└──> cd waves/

┌────[root@xps-m1710]───────[08:19:19]───────[...hare/backgrounds/waves]──
└──> ls -ltr
total 11236
-rw-r--r-- 1 root root 6240 2008-04-11 01:12 waves.xml

A simple waves.xml would look like this:

86400.00

/home/foo/background.jpg
/home/foo/background.jpg
/home/foo/background.jpg
/home/foo/background.jpg

Alternatively, HackTUX posted a way to change the background with the ever-so-simple gconf-tool2 (a "\" means line is extended, the entire command below, without the \ at the end, should all be on one line):

su -c 'gconftool-2 --direct --config-source xml:readwrite:/var/lib/gdm/.gconf -s \ --type string /desktop/gnome/background/picture_filename /home/foo/background.jpg' \ gdm