To make it short: Canonical had to disable the user's background/wallpaper on the login screen due to security issues (I can't find the related security USN article anymore).
There are plenty of ways to enable it again with Ubuntu 18.04, Unity and LightDM, however from a security perspective it is shooting a hole in your system.
Has Canonical planned to provide at some point a user-friendly way to display the user's background/wallpaper again on the Gnome login screen instead of having to apply dangerous security tweaks directly in config files?
