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 the featured image, you need to right-click on the image you want to set as the featured image and click the "Set as Featured Image" button.
Alternatively, you can click on the image and click the "Set as Featured Image" button in the image toolbar on the right ide of the gallery.
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".