The images that are shown depends on a number of settings. The most important factor is whether the catalog is shown at model or style level.
1. Go to the settings module, click on the ‘catalog’ tab, and choose the option 'View catalog as ....'. When viewing the catalog at model level, the main image is the model image, and if there is no model image, then the first style image is the main image. When displaying the catalog at style level, the main image is the first style image of the style (any model image will not be used).
2. To view thumbnails, go to the settings module, click on the ‘catalog’ tab, and choose the option 'show thumbnails'. You have two options to choose from.
3. Show only products with stock: go to the stock module and choose the option 'do not display products without stock'.