OS X Yosemite sets the login window to the last admin users background. It’s using a cached file located in: /Library/Caches/com.apple.desktop.admin.png If you replace this file, it MUST be a PNG file. If you want to lock the file so it sticks for all users, change the permissions to 555 chmod 555 /Library/Caches/com.apple.desktop.admin.png
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