Wallpaper changer

Sick of seeing the same wallpaper over and over again?
You have a lot of nice raytracing pictures and would like to see all of them from time to time?

Change your wallpapers easily with any of the following applications.

Ubuntu

Wallpaper Tray
The application lets you add directories, wallpapers are then randomly selected at login or at a given interval. You can also search your wallpaper collection, and delete wallpapers you don't like anymore.
The tray icon changes to the actual image set as wallpaper.
This one is my first selection for changing wallpapers.
Install with

sudo aptitude install wallpaper-tray

or Synaptic.

Desktop Drapes
I used this first and has the advantage that it shows if a wallpaper fits well to your chosen screen resolution.
It's big problem is that it will not include all pictures when you add a folder to your wallpaper collection. It only supports one folder for images, but you can add single pictures as well.
Install with

sudo aptitude install drapes

or Synaptic or Applications main menu.

Webilder
Webilder delivers stunning wallpapers to your Linux desktop, directly from Flickr and Webshots. You choose what keywords (tags) to watch for, and photos are automatically downloaded to your computer. Webilder can also change the wallpaper every few minutes.
I haven't tried this one yet...

Automatic startup

Any of these applications won't start automatically when restarting your session.
Therefore, you'll have to create an entry in the list of startu programs which can be found in System-->Preferences-->Sessions
Have a look at my tutorial for the Advanced Window Manager for an example.