How do I set up the login screen background in gdm?

edit

Hi,

I'm new to Fedora (ver17, x86, gnome 3) and I'm having trouble with log-in screen background set-up:

[]$ cd /usr/share/backgrounds/gnome/
[]$ ls
Aqua.jpg            GreenMeadow.jpg           Terraform-blue.jpg
Blinds.jpg          Spaceflare.jpg            Terraform-green.jpg
BlueMarbleWest.jpg  Spaceflare-nova.jpg       Terraform-orange.jpg
FootFall.png        Spaceflare-supernova.jpg  TwoWings.jpg
FreshFlower.jpg     Stripes.jpg               Wood.jpg
Garden.jpg          SundownDunes.jpg          YellowFlower.jpg
[]$ su -
Password: 
[]# su - gdm -s /bin/bash
-bash-4.2$ `dbus-launch | sed "s/^/export /"` 
-bash-4.2$ GSETTINGS_BACKEND=dconf gsettings set org.gnome.desktop.background picture-uri "file:///usr/share/backgrounds/gnome/Stripes.jpg"

** (process:1527): CRITICAL **: unable to create '/run/user/eskobar/dconf'; dconf will not work properly.
-bash-4.2$ ^C
-bash-4.2$