Solution for changing Big Sur login wallpaper to custom.
Just sharing a solution I've found on Stack Exchange. Credit to whoever discovered it.
- System Preferences > Users & Groups.
- Open lock to make changes.
- Right click on user in left column > Advanced Options.
- Copy UUID value.
- Go to /Library/Caches/Desktop Pictures.
- If it doesn't exist create Desktop Pictures folder.
- Inside Desktop Pictures create folder with UUID value as name.
- Right click on folder > Get Info.
- Open lock to make changes.
- Grant permission to Read & Write to user, admin, everyone.
- Make sure FileVault and Guest user are deactivated.
- Change desktop wallpaper.
- Restart computer.
Big Sur will now always create a lockscreen.png file in the UUID folder matching the current wallpaper.
Tested on a 2018 MBP and works perfectly.
Enjoy!
MacBook Pro 15″, macOS 11.0