Unity greeter background not changing for users automatically after upgrade

Solution 1:

If I understand you correctly, you want the desktop background for the login screen to change depending on which user is highlighted.

Yes, it does do this by default (it sets it to the one the user uses). The caveat is that since you upgraded, it doesn't automatically set this for existing users.

Simply change the wallpaper for each user, then change it back to what you had before. This will update the login screen to show the correct wallpapers. I had to do this with network users.

Solution 2:

Recently I ran into the same problem and discovered there are two key settings that need to be enabled for this to work.

1. Make sure your wallpaper is readable by lightdm.
2. Enable unity-greeter background drawing. Run the command gsettings set com.canonical.unity-greeter draw-user-backgrounds true.
3. Enable the background plugin from the accounts daemon. Run the command gsettings set org.gnome.settings-daemon.plugins.background active true. (This will have the accounts-daemon process update the /var/lib/AccountsService/users/johndoe file with John Doe's wallpaper location.)

I had to update these commands a couple of times for my wallpaper to get written to the respective file and then unity-greeter started showing my background again :)