Post output of ls -la /home/$USER | grep root please … this might reveal something. Maybe some files &/or directories in your now modified /home have wrong ownership. I have never attempted installing a second DE so there are some unknowns including this one. Never a good idea IMHO due to potential conflicts. 
Also, have you put those wallpapers in one of the default directories or are they maybe in one which doesn’t get mounted before the wallpaper is supposed to be loaded and displayed?