Take a look at this: http://fixunix.com/setup/397585-fedo...in-screen.html
It says that GNOME does not have that option, but KDE has.
This what I am writing now should not be done if you are not skilled user! Be warned! You might not be able to login back to yous system!
There is little picture in /boot, file named "message". I think that one is for user list (if it is enabled).
The splash screen on login is in this archive: /boot/grub/splash.xpm.gz. If you find compatible splash archive, you could just rename the file (do NOT delete it!) and copy another one you like.
But first, try login to KDE and try there.