Enter the following command in a terminal:

gksu gedit /etc/lightdm/unity-greeter.conf

In the Text Editor window that opens make sure you edit the line highlighted in the screenshot below: -

Edit this path to lead exactly to the location of the image you wish to use. Remember – it must end in a image format (.jpg, .png, etc)

If you set it to the same image as your desktop wallpaper you'll get a seamless-looking login. That just looks great. Of coarse that only happens for other users if they use the same wallpaper. Save the file, log out of your session (no need to restart) – You’ll now see your custom LightDM background in use!