1. Move your favorite login wallpaper to your Home folder. Make sure that it is of .JPG format.
  2. Move the wallpaper to the system wallpaper directory. In the terminal:
    sudo mv ~/your-wallpaper-name.jpg /usr/share/backgrounds
  3. Activate the Appearance window upon login
    $ sudo cp /usr/share/applications/gnome-appearance-properties.desktop /usr/share/gdm/autostart/LoginWindow
  4. Close the terminal. Log out of your current session.
    At the login screen, the Appearance window will show up. Go to the background tab and select your favorite wallpaper as the background. (If you can’t find your favorite wallpaper, click Add. You should be able to find your wallpaper in the /usr/share/backgrounds directory).
  5. Login again and remove the gnome-appearance-properties.desktop file that you copied in the previous step. This will prevent you from getting the appearance-properties windows at every gdm login.
    $ sudo rm /usr/share/gdm/autostart/LoginWindow/gnome-appearance-properties.desktop

Sumber : http://nfolamp.wordpress.com/2010/05/18/change-the-login-screen-background-in-ubuntu-10-04/