This is a little bit the dirty way but makes you love the terminal, so open a terminal and type the command below;
sudo nano /etc/lightdm/unity-greeter.conf
we only need to edit the “background=/usr/share/backgrounds/warty-final-ubuntu.png”, I would prefer to put a comment before the background= so it would look like this;
#background=/usr/share/backgrounds/warty-final-ubuntu.png










