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. See the man page for a complete list of features.
zufall is targeted at the ever dwindling group of users who roll their own desktop environment by combining a window manager like sawfish, ratpoison, i3 or Fluxbox with other programs (docks, terminals, file managers etc). It does not work if the X root window is hidden by another program that does not pick up changes to the root-window. This is the case in all modern „normal“ desktop environments.I wrote zufall because chbg, the program I used before, was the only GTK1-dependency left on my desktop. Also, I really disliked the background-colors chbg's "random-background"-feature gave me most of the time. Maybe it's useful to you.
The latest version was released May 2016.