What you are looking for, if I've understood you correctly, is a change in the gdm theme. Please go to http://art.gnome.org,
select Themes>GDM Greeter, choose download the ones you like. Later on, as root, open a console and type gdmconfig, then choose the graphical login section and install the themes. Please note, you need not untar/unzip the themes, they are installed from the gz or bz2 files directly.
Hope this solves your problem.