Wallpaper change automatically

this I found on http://stackoverflow.com/questions/2182468/run-cronjob-as-user-to-change-desktop-background-in-ubuntu, perhaps use cron to run it...

#!/bin/bash

# Script to randomly set Background from files in a directory

# Directory Containing Pictures
DIR=/usr/share/backgrounds
LOG=/home/username/background.log

# Command to Select a random file from directory
PIC="$(
for p in [jJ][pP][gG] [pP][nN][gG] [sS][vV][gG] ; do
 ls $DIR/*.$p
done | shuf -n1
)"
# Command to set Background Image
if [ -z "$DBUS_SESSION_BUS_ADDRESS" ] ; then
 # this is because of gconftool bug in cron
 TMP=~/.dbus/session-bus
 export $(grep -h DBUS_SESSION_BUS_ADDRESS= $TMP/$(ls -1t $TMP | head -n 1))
 echo $DBUS_SESSION_BUS_ADDRESS >> $LOG
fi

gconftool-2 -t string -s /desktop/gnome/background/picture_filename "$PIC"