The catalog will properly display your images if:
1. You have 1, 2 or 3 images.
2. The large image is missing, the pop up window will display the Medium image, or Small image if that is the only one available.
3.You have at least a small image; you will display an image in the category column, in the product description, and in the pop up window. If you have images optional, and have no image, that's okay too. The products will display with no broken image and no pop up window available.