I have researched this but never got around to try it. In 10.10 and 11.04 we could choose one of two background slide shows. This is how it was done and you can copy the method.
Go to /usr/share/backgrounds and there you will find the 11.10 background images and a folder called contest. In the folder called contest is a document called background-1.xml. That is the script that runs the background slide show. Here is a sample of what it looks like:
<!-- This animation will start at midnight. -->
The rest of the script is a repeat of these lines but point to and from the different images in the backgrounds folder.
You now need to
1) create another folder with a name of your own choosing in the backgrounds folder.
2) copy your chosen background images into the new folder.
3) copy the document background-1.xml in that new folder.
4) edit/modify the background-1.xml document to point to your background images which should be in this same new folder. so, the paths to the images will be different. Instead of
It will be
Then you should get another background slide show option in the Appearance utility.
I have saved the backgrounds and the scripts from 10.10, and 11.04, so I know that this is the way it is officially done.
You will need administrator privileges to copy and edit these scripts. So, you need to run
gksudo gedit /usr/share/backgrounds/contest/background-1.xml
That will open the document in Gedit with administration privileges which will let you edit it and save it in the new folder. And
To give the file manager administration privileges so that you can create the new folder.
EDIT: Further research shows that there is another script involved. It is found at /usr/share/gnome-background-properties, and it is called ubuntu-wallpapers.xml. This is the script that tells the Appearance utility what images to put in its left panel and which folder to look in for the script background-1.xml to use as a slide show. The script needs to be edited following the pattern.
You may find as I did that the left panel ends up with duplicate images or folders. This is because a duplicate ubuntu-wallpapers.xml document is created when you edit the original one. Just move the duplicate ubuntu-wallpapers.xml document from the gnome-background-properties folder to the rubbish bin.