GDM (Gnome Display Manager) is the graphical login program which you see when you log-in to your Linux computer. GDM used to support themes and a lot of other customizations - all available through the "Login Screen" option under System > Administration - but the new GDM2 (2.28) which comes with Ubuntu Karmic for instance, doesn't support all this unfortunately. But an Ubuntu forums user has created a basic GDM configuration tool