defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture [path]

Title		Placeholder
Bundle ID/Path		Display Bundle
Key		Display Group
Default value		Options	kCFPreferencesCurrentHost kCFPreferencesAnyUser Keypath
Datatype	Boolean Boolean (Negate) Integer Float String Bundle Identifier Font Name File Path Rect Array Array (Add) Array (Multi Add) Dictionary Dictionary (Add) Date Color URL	Units	Widget Popup
Description	The picture shown in the background when the login window is displayed	Flags	Verified Hide from UI Dangerous For Debug Broken Has Application UI
Values (as plist)		OS Version	to
		App Version	to
Notes
This change will affect all users.

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