After uploading an image to a product in the Magento backend, your image will be displayed on the product detail page in the “more views” block.
Normally your base image will also be listed here. Often people do not want to show this image again in the “more views” block. To prevent this, you will have to exclude the image from the more views. To do this you simply check the checkbox “exclude” on your image you indicated as your base image: