This is not a very big task just copy these comand on to your terminal and you can change the background picture of you GDM login screen from the default to your favorit wallpaper follow the steps.
Step 1.
Open the terminal Applications -> Accessories -> Terminal and type $gksudo nautilus (This opens nautilus in root mode) and press enter
Step 2.
Copy your picturs to /usr/share/backgrounds after that close nautilus
Step 3.
In the terminal enter this command $ sudo cp /usr/share/applications/gnome-appearance-properties.desktop /usr/share/gdm/autostart/LoginWindow
Step 4.
Now Log Out
Step 5.
Change the background to the one you like and log in to the system
Step 6.
Open the terminal and enter this $ sudo unlink /usr/share/gdm/autostart/LoginWindow/gnome-appearance-properties.desktop (This will disable the re-prompting of the background change dialog when you re-log in to the system)
OK, Thats it you have your favorit picture as your background when you log-in.
