Gnome Lock Screen Background?

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http://forums.fedoraforum.org – I have installed Fedora 18 64-bit on my laptop. A many thanks to the fedora team for making it such smooth and beautiful release. I want to change the lock screens background image. The background image of the lock screen is by default set to match the user's wallpaper. But I want to set it to use image that is different than my desktop wallpaper. Any clues on how to do that? (HowTos)