This is a very simple script I did for changing the background in periods of time. The script was tested in Ubuntu 12.04.

bgchanger.sh

#!/bin/bash
#
# Copyright 2013 Ian Liu Rodrigues <[email protected]>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

# Rationale
# =========
# This is a very simple script for changing the desktop background in periods
# of time.

# Installation
# ============
# 1. Give execution permission to this script:
#   chmod +x bgchanger.sh
#
# 2. Move it to a folder in your home:
#   mkdir -p ~/.scripts && mv bgchanger.sh ~/.scripts
#
# 3. Create a startup application to run this script:
#   https://help.ubuntu.com/community/AddingProgramToSessionStartup
#
# 4. Almost done! Now you need to configure the script, see below.

# Configuring
# ===========
# You can modify the following variables to suit better your needs:
#    (default values are in brackets [])
#
#    DIR                                       [$HOME/Pictures/Wallpapers]
#             The directory where the wallpapers are.
#
#
#    PERIOD                                                          [20m]
#             The interval of time in which wallpapers are going to change.
#             This value is passed directly to the shell command sleep. See
#             its man page for the time format.

LOCKFILE=/tmp/bgchanger-$HOSTNAME
if ! ln -s `basename $LOCKFILE` $LOCKFILE ; then exit ; fi
trap 'rm -f $LOCKFILE ; [[ $sleepid ]] && kill $sleepid ; exit' SIGTERM SIGHUP SIGINT SIGABRT SIGQUIT

DIR=$HOME/Pictures/Wallpapers
PERIOD=20m

# ============================================================================
# Only change the lines below if you know what you are doing! :)
while true ; do
  sleep $PERIOD & sleepid=$!
  wait

  FILES=(`ls -1 $DIR | grep -v ^_`)
  MAXNUMBER=${#FILES[@]}
  number=$(($RANDOM % $MAXNUMBER))
  file=file://$DIR/${FILES[$number]}
  echo "Changing background to $file"
  gsettings set org.gnome.desktop.background picture-uri "$file"
done