There seems to be no easy way to disable the desktop window that covers up the root-window. (Maybe add checkbox preference in desktop background preferences?)
whoops, don't know why this one slipped by. if you don't want xfdesktop to start, either comment it out in your ~/.xfce4/xinitrc (may need to copy from /etc/xfce4 and make executable), or, if using xfce4-session, kill it ('killall xfdesktop') and save your session.