I recently installed XFCE GUI, because I was interested in what it might look like. This changed the wallpaper on my login screen to the standard XFCE greeter.
When I switch back to GNOME, the wallpaper doesn't change back to what it was before, which was the default Ubuntu wallpaper.
I followed the instructions of this post: How do I change the login screen in Ubuntu 16.04+? on how to change the login wallpaper, but to no effect. If I open LightDM, the image selected there is actually the default wallpaper of Ubuntu and I also verified this by going into the folder and looking at the image files. It seems that the XFCE setting is somehow taking precedence over all other settings. I considered uninstalling XFCE, but I would still like to test it.
I have now also completely removed XFCE from my system using this guide: https://www.howtoinstall.co/en/ubuntu/xenial/xfce4?action=remove, and I still can't switch or even get rid of the XFCE greeter wallpaper.