As many Fedora 14 users have probably found out, you can no longer change the default background (Log in screen background) easily.
Why they removed this feature, I don’t know. But here is probably the easiest way to change the background:
Step one: Run gconf-editor as root.
Step two: Press Ctrl+D or go to File > New Defaults Window
Step three: In the default settings window, modify the key: "/desktop/gnome/background/picture_filename" and set it's value to whatever the location of your background is.
Step four: Restart your computer
That should do it!