I have been trying to change my login screen wallpaper to one of my own using ubuntu-tweak. The wallpapers are located in Ubuntu's default wallpaper folder and I have given all users read privileges on them.
Now there isn't even a wallpaper left on the login screen.
Best Answer
By trying the answer of Senio I had no luck. But with little modification I hit the jackpot :
path-to-image is actually the path where the image you want as log in screen wallpaper is stored e.g. : /usr/share/backgrounds/x.jpg
I tested it and it's working
I found this page which offers some tips regarding this problem : Ubuntu13.04 LighDM