The Windows logon background is a file called "backgroundDefault.jpg" and should reside in the path: "c:\windows\system32\oobe\info\backgrounds\"
The full path may not already exist so you may need to create it.
Create an appropriate .JPG image, and save it as "backgroundDefault.jpg" in the above path.
Now, in order for it to show up, you need to set a registry key. Simplest way is to paste the text below into a text file and save it as "set_background.reg". Double click on it and it should be imported automatically.
Now press Ctrl-Alt-Del and you should see your new background. Press Esc to go back.