Somehow, someway, the image at path /System/Library/CoreServices/DefaultDesktop.jpg was either moved or renamed. The default image can be found in /Library/Desktop Pictures/Nature/Leopard Aurora.jpg. If you make a copy of the the Leopard Aurora.jpg and rename it to DefaultDesktop.jpg and put it in that folder, restarting the computer should restore your expected background at the login window. Note that you can actually use any image as long as it is titled DefaultDesktop.jpg and has that path.
Other customization options for the login window.