This video shows you how to change your login background.
The login wallpaper is stored in:
Macintosh HD -> System -> Library -> Frameworks -> AppKit.framework -> Resources ->
NSTexturedFullscreenBackground.png
You will need to replace the “NSTexturedFullScreenBackground.png” with your own image. It will need to be the same screen resolution as your Mac. To find out your screen resolution open “Settings -> Display”. If required, you can resize the image in Preview. I would recommend that you keep a backup of the old background incase you need to revert back later.
If you find a great background leave a comment below.