WP-Ecommerce and WPOLS are two completely different stores.
There are two types of images in WPOLS; thumbnail and main product. The thumbnail will appear on the category page (where you have a list of products) and it will appear on the product page if you have no main product image uploaded.
You can upload multiple main product images. These will be displayed on the product page as one large image and the rest as smaller images. When the customer clicks on a smaller image it will replace the large image. If the customer clicks on the larger image it will expand to full size.