Each product can have one main image and any number of additional detailed images. The thumbnails of the detailed images are shown below the right panel below "Add to cart" button on the product page.
1. Go to your Control Panel > Catalog > Choose your desired product
2. Open Gallery tab and upload all your images. They will be shown on the product details page.
When a customer opens a product page, he/she sees the thumbnails of all additional images. When any thumbnail is clicked, your store shows a popup with a bigger image. All images are automatically resized to fit the size of the customer`s browser window which allows to see the whole image without the need to scroll the page.
If the customer clicks a re-sized image, the image will be displayed in a new browser tab in its original size. If the image already fits the browser window it will not be resized.