How to add/remove login desktop image

The registry folder that handles the login desktop image is:
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Setup

Edit the key 'Wallpaper' and add the full path to the image you want to display or leave blank to remove.

Having an image at the login screen can be very frustrating if you try to do remote support over a slow link at the whole image has to be downloaded before you can login.