Sun 13, Oct 2019 macos
Changing the login screen wallpaper in macOS Mojave is very different from previous macOS versions.
In older versions, you could simply change the file
/Library/Caches/com.apple.desktop.admin.png, but this doesn't work anymore in Mojave.
Instead, it loads the login wallpaper from
/Library/Desktop Pictures/Mojave.heic. So, in order to change the login screen wallpaper, you'd have to replace the file with your custom login wallpaper.
I would recommend renaming the original file to something different (like
Mojave_Original.heic), so you don't lose the standard wallpaper.
If you are using JPEG, PNG or anything else than HEIC for this wallpaper, make sure you convert it correctly. I tried it using GIMP, but that just gave me a black background instead of the actual new login wallpaper.
After that, I tried it with jpeg2heic.com, which worked perfectly fine.
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