If you only have one image for a product you should load it in the Product Thumbnail slot. I know this sounds backwards but the way that X-Cart handles images, if there is only one image for a product if falls back to the thumbnail image. This is the image that will be shown anywhere, not matter what, so upload your larger master image there.

There may be instance where you would still want to use both the product thumbnail and the product image even while using this mod. If you want a different image shown on the category pages than on the actual product page, then you would load an image in both slots. Some people will use a cropped (closer) view for the thumbnail slot and the full image for the product image slot. They will do this for style purposes.

So in this instance, when viewing the category pages, you will see cropped, closer views and then on the product page itself, you will see the full image and the main image. Then down below they will choose to show small thumbnails of each (within the setting of the Dynamic Image Mod). The result is the main image that will popup on click and then two small thumbs, one of a detail view and one of the full image, that will also popup on click.

This is an effective method when showing something like jeans. For the category page you can have closeup details of the jeans pockets and then on the product page you can show the full jean from the front side. Then you would have popup images of both the full jean and the pocket. Then if you have even more images that you wanted to show, they would be uploaded as the Detail Images.