Where is the wallpaper you are using stored? is that file accessible to other users? the lightdm users can only display files it can read, if your user directory access is restricted that doesn't work, it's a known issue
Where is the wallpaper you are using stored? is that file accessible to other users? the lightdm users can only display files it can read, if your user directory access is restricted that doesn't work, it's a known issue