Original image from www.muktware.com.
To change the Lightdm background you just need to do the following steps:
1 – Edit the /etc/lightdm/unity-greeter.conf (use sudo to get root privileges)
2 – Change the background=/usr/share/backgrounds/warty-final-ubuntu.png line putting the location of the new background image.
3 – Save your changes.
4 – Logout and login again.
5 – Be happy.
Original image from www.linuxnov.com