How to change a gallery's thumbnail
When image gallery pages are grouped into a “thumbnail grid” page group, each page is represented by a thumbnail image.
By default, the first image of each gallery is used as that gallery’s thumbnail, but we realize sometimes that’s not ideal.
You can choose any image you like to server as the thumbnail by clicking the “star” icon at the bottom left of each thumbnail.
You can also hide the image by clicking the “eye” icon at the top left, if don’t want your thumbnail image to be displayed within the gallery itself.