MacOS – Cannot change login wallpaper in 11.x Big Sur

big surmacos

Similar to the question here, is there a way to change the login background wallpaper in Big Sur? During the beta, I was able to do it via a combination of the procedures in Catalina and the bless command described at https://eclecticlight.co/2020/06/25/big-surs-signed-system-volume-added-security-protection/

This doesn't seem to work with 11.0.1 (20B29). It keeps reverting to the com.apple.os.update-779B* snapshot. Note that I am using FileVault.

Thanks

FileVault has to be OFF, to use a non-system sealed shapshot to boot Big Sur

Here's a simplified version of what you need to do (thanks to user3439894):

Put an image you want to use as login background somewhere in your file system (I used my Downloads folder in this example).
Reboot your Mac holding Cmd+R to boot into Recovery OS.
Open Terminal from the Utilities menu on the menu bar.
Backup/Rename Catalina.heic to e.g. Catalina.original.heic in /Volumes/Macintosh\ HD/System/Library/Desktop\ Pictures/, e.g.:
```
cd  /Volumes/Macintosh\ HD/System/Library/Desktop\ Pictures
mv Catalina.heic Catalina.original.heic
```
Copy your desired image: cp /Volumes/Macintosh\ HD\ -\ Data/Users/[short_user_name]/Downloads/[image_from_step_1].jpg /Volumes/Macintosh\ HD/System/Library/Desktop\ Pictures/Catalina.heic
- Note you can replace the destination path in the above command with just ./Catalina.heic as you should already be in the target directory if you performed the previous step to backup the original file.
Reboot into normal mode.
Change any option in System Preferences > Users & Groups > Login Options to change the cached image.

diskutil unmountDisk force DISK_PATH (or whatever your disk is)