Because the example images are set up for the category id's in the demo products package, and your categories don't include any of the same id's.

If you rename some of the example images to category id's that you have on your site, you will see them replacing text.

The readme includes descriptions of how each modification is done in the code, but you don't need to study that part. Concentrate on the directions for naming images and the like, and don't worry about the modifications that you are not interested in using.