Taking a set of images and making a image pipe out of them is a common configuration. The easiest way to do this is to create each indivual image as a separate layer in an image, much like you would if you were going to save an animated image. And in fact this is often a convient way to think of image pipe creation. As designing an animation of which snapshots are going to be composited to the canvas. Let's start with a simple example image, an 8 layer image. In this case, with the numbers 1-8 on each layer. This will help to make it more obvious what the various "ranks" do.
Let's say we save the image above as an image pipe with one dimension, four ranks and random selection. One of the images will be chosen for the brush at random. If we paint a short line with it (spacing 150), we get something like image above.
