- Get a copy of the image you would like use.
- Name it com.apple.desktop.admin.png
- Copy it to /Library/Caches
- You should probably backup the original in here if you ever want to revert to it.
- Logout or reboot the machine.
- All done!
I've made an utility that can be installed on machines to point them to a specific image saved on a web accessible method to set them all to that same image on the login window. This is still a work in progress, so be careful which machines you install it on.