the "standard" login screen background on OSC (Leopard and Snow Leopard) can be modified using these command line incantations:
cd /System/Library/CoreServices sudo mv DefaultDesktop.jpg DefaultDesktop_org.jpg sudo cp /path/of/image.jpg DefaultDesktop.jpg
I learned that here
QUESTION: is there a GUI based utility for people like my wife and daughter?
.. they DO want to customize (worse: they want me to do it ),
but they DON'T want to learn the terminal ( since I know how).
They DO know how to enter an admin's credentials when prompted.