The first page that your client sees when they load their Gallery is the Gallery cover. This is the introduction to the Gallery - the "book cover" to your photos. You can set any photo in a Gallery to be the image shown on the Gallery cover.
By default, the first photo in a Gallery is chosen as the featured image.
To set your featured image you can do it in one of two ways.
You can click the image you want to be the cover image and click on the star icon under the "upload photos" button.
You can right-click on the image you want to use and click the "Set as Featured Image" button.
To adjust the crop of the featured image, you will be prompted to do so after selecting it as your featured image. Click and drag the focal point and click "Save".