zufall is a simple program to change the background periodically. It takes one or more directories with images as a parameter (no GUI) and sets a randomly chosen image as background. If there is a border remaining, its color is the average color of two corners of the image. This mostly fits the image very well. It doesn't work if the root window is hidden by another program, like in Gnome or KDE. However, it is possible to execute a configurable command to effect the change whenever the image is supposed to change. A script to do this in Gnome is included.