Approximately 7 out of 10 articles will tell you to overwrite the image file that the OS uses as the login screen background by default. Of course this is not the correct way to go since this image is in the /System subtree (
/System/Library/CoreServices/DefaultDesktop.jpg to be precise) and theoretically a system update could write back the original image. Instead set the proper system preference to point to the image of your choice:
defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture "/Library/Desktop Pictures/Jaguar Aqua Blue.jpg"
To revert back to the default behaviour you've to delete this preference since it was not there originally at all:
defaults delete /Library/Preferences/com.apple.loginwindow DesktopPicture
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