1. Go to the product page
2. Do right click on the product image and inspect
3. Select the div of your main product image and then got either the CSS class or ID of the div.
Example: In our case it's
4. Now add this CSS class in the admin field. Eg. .media or .product
Please, add only one CSS class of the div or the ID of the div
In our case, we will add only: .media