I already published a script that sets a random image file as desktop wallpaper. It has only one downside: you have to run it every time you want to rotate your wallpaper.
However GNOME since 2.28 allows assigning a slide show as wallpaper by providing an image list in XML format. There you can specify the order and duration for each image. Moreover, you can even define a transition from one image to another.
So I decided to create a simple script that generates an XML file for a given set of images and, optionally, sets that as the current wallpaper. It requires GNOME 3+ or Ubuntu (11.10 or later).