From what I understand, Ubuntu MATE switched to Slick but still uses LightDM (?). Is Slick just a greeter or a different login manager that somehow uses LightDM’s configuration?
Anyway, I can’t make it display the users background despite the option checked in the preferences (and the slick-greeter.conf having draw-user-backgrounds set to true). Not sure if this is the same issue that was already present in UM with LightDM or a new one but my old solution (the script that I made) doesn’t seem to work anymore.