If you have multiple photos on a product page, you can change their display order.
Changing the Order of Product Photos
In your Admin Area, open the Products feature and select your product. Locate the image section.
Hover your mouse over an image and use the right and left arrows within the orange edit bar to change its position.
Click the orange Save button at the top left to save your new product photo order. The new order will be reflected on your storefront.