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.
By trying the answer of Senio I had no luck.
But with little modification I hit the jackpot :
sudo -i
xhost +SI:localuser:lightdm
su lightdm -s /bin/bash
gsettings set com.canonical.unity-greeter draw-user-backgrounds 'true'
gsettings set com.canonical.unity-greeter background 'path-to-image'
exit
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
