Here is a login interface management utility to change your GDM. It allows you to set a wallpaper/picture as login background, change the autologin option, prompted or userlist login, etc.
To add gdm2setup to your Synaptic Package Manager open a Terminal window (Applications -> Accessories -> Terminal) and copy+paste the following line:
sudo add-apt-repository ppa:gdm2setup/gdm2setup
To install gdm2setup copy+paste the following lines:
sudo apt-get update
sudo apt-get install python-gdm2setup
gdm2setup will now update along with your other packages automatically. When installed you can find gdm2setup under System -> Administration -> Login Screen(GDM2Setup)