[jmax@gir] ~ $ cat ~/Desktop/Autostart/backdrop_renew.sh 
#!/bin/bash
#
# requires that you have a list of images in ~/.config/xfce4/desktop/backdrops.list
# prefixed with "# xfce backdrop list" on the first line
handle_hup ()
{
          exit 0
  }

  trap "handle_hup" HUP

  while(true)
  do
          sleep 60s
          xfdesktop -reload
  done 
[jmax@gir] ~ $ cat ~/backdrops/update.backdrop 
#!/bin/bash
# XFCE4 backdrop auto-update
( echo "# xfce backdrop list"; find ~/backdrops/ |grep jpg |grep -v lol ; ) > ~/.config/xfce4/desktop/backdrops.list