How to Change Your Login Screen Wallpaper on Ubuntu.

Start the terminal and run this command to get root privileges:

$ sudo -i

Allow now lightdm to establish a connection with the Xserver with this command:

$ xhost +SI:localuser:lightdm

Sign in now as lightdm:

$ su lightdm -s /bin/bash

Disable now “draw-user-backgrounds” and “draw-grid” with these commands:

$ gsettings set com.canonical.unity-greeter draw-user-backgrounds 'false'

$ gsettings set com.canonical.unity-greeter draw-grid 'false'

You can now set your custom wallpaper with this command:

$ gsettings set com.canonical.unity-greeter background '/path/to/wallpaper.png'

Replace “/path/to/wallpaper.png” with the full path to your wallpaper. You can now logout and test your login wallpaper.

If you prefer to restore default settings, run this sequence of commands:

$ sudo -i

$ xhost +SI:localuser:lightdm

$ su lightdm -s /bin/bash

$ gsettings reset com.canonical.unity-greeter draw-user-backgrounds

$ gsettings reset com.canonical.unity-greeter draw-grid

$ gsettings reset com.canonical.unity-greeter background

$ exit

$ exit

Restart now your computer.

Enjoy!