DISCLAIMER:
You MUST have a password on your account
This will NOT work if your Mac has multiple users accounts. (not even a guest account)
This MIGHT work with an upgraded OS to Big Sur but not guaranteed.
For best results, perform a clean install of Big Sur from a USB stick, not an upgrade, and only have one account on your computer when setting it up.
Issues have been reported to not work on 11.2, Apple seems to have blocked this exploit.
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I noticed many threads online about how complicated it is to change the Big Sur lock screen wallpaper, there are guides showing how to go in terminal as root, disable tons of security features, etc.
It’s all just far too complicated for how easy it really is.
This is how to change the lock screen wallpaper on your Mac running macOS Big Sur or earlier.
If the lock screen background will still not change for you, try this and then repeat the steps here.
https://youtu.be/PUZZuKlKurI
Note: Frozen North Galaxy is the original author of this video, we just embed it, if you have any questions please contact him via Youtube.