when setting the wallpaper from the terminal still does change the wallpaper but you will not be able to double click and or open pcmanfm directly. either from the double clicking on the icon or from the terminal directly.
no errors are reported from the terminal when trying to launch pcmanfm just returns to the prompt but nothing opens
like i said this only occurs after you changed the wallpaper from the terminal
however a clue may exist. i noticed that after you change the wallpaper from the terminal you can't launch pcmanfm from the terminal but you can launch it as root..
hope that helps