Mac OS X has only one background image for its Login Screen by default, but the commands below can be used to change this image.
Your new background image should be a 2560 × 1600 pixels JPEG image.
To change the Login Screen’s background image, open a new Terminal window and enter the following command (replacing “/path/to/backgroundimage.jpg” with the full path to your preferred background image):
defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture "/path/to/background/image.jpg"
To revert the change, open a new Terminal window and enter the following command:
defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture "/System/Library/CoreServices/DefaultDesktop.jpg"
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