Here is a guide to install Variety wallpaper changer on Ubuntu 13.04. You can install Variety by adding Peter Levi’s PPA for Variety.
Variety is a wallpaper changer for Ubuntu which is featureful, yet slim and very easy to use. It can automatically download wallpapers from various online sources, allows rotating them on a regular interval or on demand, and provides easy to use ways to separate the great images from the junk. Notable other features include: showing a nice digital clock on the desktop, option to apply various interesting image filters to the wallpapers, great visualization of the history and the image folders which allows the user to very easily change the wallpaper manually. Visit the Variety Homepage.
To add the PPA and install Variety open a terminal window (press Ctrl+Alt+T) and copy+paste the following lines:
sudo add-apt-repository ppa:peterlevi/ppa -y
sudo apt-get update && sudo apt-get install variety -y
When the installation is finished you can close the terminal window and to start Variety press the SuperKey and type: variety
When Variety is installed and you start it for the first time an Variety indicator will be added to the indicators in the top menu. You can add/remove and start/stop Variety using this indicator.