Solution for changing MacOS Big Sur login wallpaper to custom

1. Go to System Preferences > Users & Groups
2. Click the lock to make changes
3. Right click on user in left column > Advanced Options
4. Copy UUID value
5. Go to Macintosh HD/Library/Caches/Desktop Pictures
6. If folder doesn't exist create Desktop Pictures folder
7. Inside Desktop Pictures create folder with UUID value as name
8. Right click on folder > Get Info
9. Open lock to make changes
10. Grant permission to Read & Write for user, admin and everyone for both the UUID folder and Desktop Pictures folders
11. Make sure FileVault and Guest user are deactivated
12. Change desktop wallpaper
13. Restart your computer

MacOS Big Sur will now always create a lockscreen.png file in the UUID folder matching the current wallpaper.

Tested on a 2018 MacBook Pro and works perfectly.