Reboot system in recovery mode by holding command + r on startup.
Open Terminal in recovery mode and enter the command “ disable”.
Reboot the system and login as an administrator.
Open Terminal and enter the command (see note below) “sudo mount -t apfs -wu /dev/disk1s5/Volumes”.
Enter the following command: “sudo mount -wu /; killall Finder”.
Navigate to Computer -> Macintosh HD -> System -> Library -> Desktop Pictures.
Rename the Catalina.heic file to Catalinaold.heic or a different unique file name.
Move the desired login screen image into the Desktop Pictures folder.
Rename the new file to Catalina.heic and then log out of the account to confirm changes.
Step 4 details: Replace the “disk1s5” part of the command with proper partition name for your computer.