If you need to replace the system default image in Leopard OS X 10.5. You can do this easily from the terminal:

sudo cp /System/Library/CoreServices/DefaultDesktop.jpg /System/Library/CoreServices/DefaultDesktop.jpg.bak

Then just copy in the image you want (you really want this to be the correct aspect ratio/resolution):

sudo cp ~/Pictures/filename.jpg /System/Library/CoreServices/DefaultDesktop.jpg

Custom Login Screen