There's no real need. If you go into the settings manager -> desktop you'll see you can create lists of wallpapers for XFCE to switch between.
Originally Posted by rickympl
Once a file as has been set up, adding the command "DISPLAY=:0.0 /usr/bin/xfdesktop --reload" to your crontab as often as you want the wallpaper to change will do the trick. So, for example, the line:
will select a wallpaper at random from your list every 20 minutes.
*/20 * * * * DISPLAY=:0.0 /usr/bin/xfdesktop --reload >/dev/null 2>&1