First, you'll need to generate a slideshow file. An example can be found at /usr/share/backgrounds/contest/precise.xml . I created ~/desktopbackgrounds/ and copied the example file there:
I then modified it to suit my needs. Alternatively, you can use the script from http://nmlaxaman.blogspot.com/2012/07/generate-wallpaper-slideshow-xml-for.html . You may want to change the duration first. You'll also need to save the images you want in the slideshow to ~/desktopbackgrounds/
Finally, we need to provide a user-specific version of /usr/share/gnome-background-properties/precise-wallpapers.xml by creating ~/.local/share/gnome-background-properties/ and copying precise-wallpapers.xml there:
Finally, we need to modify that file to point to our new slideshow. Add or modify an entry as follows:
N.B. that you need to change <username> to your username.
Then, you should be able to edit the background and select your new slideshow!