When you upgrade to Mojave, Apple swaps out the background for the login screen. There's no user-facing control to change it back — you have to copy your preferred JPG (or HEIC) over the default image at `/Library/Desktop Pictures/Mojave.heic`. https://www.reddit.com/r/MacOS/comments/9iltpo/mojave_how_can_i_change_login_screen_wallpaper/e6mprzf/
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