By trying the answer of Senio I had no luck.
But with little modification I hit the jackpot :
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'
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 :