At this point in time, April 2019, we see a strange behavior on our XFCE desktop.
IF you have set your wallpaper with a wallpaper coming from /usr/share/backgrounds, you will see that the background of your lightdm login screen will change to the wallpaper of your XFCE background.
Basically take a look at the gif.
You can set whatever wallpaper via lightdm settings and it will always be overwritten.
In the video we explain why accountsservice and mugshot are in there.
How do we solve it?
Make sure you use a wallpaper that is NOT coming from /usr/share/backgrounds.
Use your own personal wallpapers to set the background from XFCE and lightdm background will NOT be overwritten.
You can change the wallpaper with variety or with Desktop settings.
You can also solve by uninstalling accountsservice. You will notice that mugshot will not work anymore.