In Fedora 15 it's actually quite simple to change the login screen. You just have to sign in as root and open up the GDM control center, then select Background and select the background you want for your login screen.

 

This is how you do it:

 

su

-u gdm dbus-launch gnome-control-center

 

If that gives you errors try this first:

 

xhost +

 

You should get the Gnome System Settings menu up. Just click on Background, then select the background image you want to use on the login screen, then close out of System Settings.