The first page 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.
There are two ways to change the featured image for the gallery cover:
Right-click any photo in a gallery while editing the gallery and click the "Set as Featured Image" button.
Select a photo in a gallery while editing the gallery and click the "Set as Featured Image" button in the top bar of the "Photos" section.
To adjust the crop of the featured image, click the "Crop" button while hovering over the featured image thumbnail (next to the "Gallery Name" in the top section while editing the gallery) to open the crop modal.