The images are autoloading based on the products_image for the small, medium, large, additional images and additional large image ...

You can use just one image, but you don't want a huge image used that gets smushed to be small, medium and large ...

You can use them as you like ... this might give you a better idea of how they work:
http://www.zen-cart.com/forum/showth...t=calicocat%2A