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After following the instructions in this post (How do I change the login screen background in macOS Mojave?), I still could not change the login screen on start up. It works when I log out, but not when the computer restarts.

Any other solutions?

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This solution can be found on this page on the link bellow at Apple:

  1. Type Command + Space
  2. Then type /Library/Desktop Pictures/
  3. Press the enter key
  4. Grab any picture you like and add the extension .heic (this will change the file format, that's what you want)
  5. I did not bother with resizing and changing the DPI (maybe it could be a necessary step for you depending on your machine
  6. Navigate to System Preferences > Users & Group
  7. Unlock the pane
  8. Enable or disable the Guest user account (in fact changing any setting in Users & Groups worked for me. This resets something for some reason)

https://discussions.apple.com/thread/8556569

If you wanted to change the lockscreen (which is different the the login screen), you'll see one or more folders inside this directory:

/Library/Caches/Desktop Pictures/

From there you simply need to rename your image to:

lockscreen.png

If there are multiple folders, you'll have to figure out which one your account is using. Ironically, you'll see the lock screen when you logout, but not when you click on Lock Screen.

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