Create Background Slideshow (CreBS) is a python script (don't get scared now, it comes as a deb with an Ubuntu PPA) which can be used to create XML slideshow backgrounds for the GNOME desktop. The XML file can then be used as an automatic changing wallpaper.
Create Background Slideshow (CreBS) 0.93 was released today with some nice features:
- Added file overwrite warning - a file-overwrite warning will now prompt you if you try to create a second slideshow with the same filename, preventing accidental loss
- Integrated with Nautilus's right-click menu - you can now right click some JPEG, PNG, GIF, or SVG images and select "Open with" > "Create Background Slideshow" from within Nautilus.
But the most interesting new feature is the addition of command-line options. Just an example: adding a command like this: "crebs -gq -d 3600 -n "Slideshow Name" /path/to/folder/*" to your startup applications will keep your wallpaper updated as you add images to the folder over time.
Install Create Background Slideshow (CreBS) in Ubuntu:
sudo add-apt-repository ppa:crebs/ppa && sudo apt-get update
sudo apt-get install crebs
Once installed, you'll be able to access it via the System > Preferences > Create Background Slideshow menu item.
For other Linux distributions, download the source from HERE.