Google Photos Album
I changed the album's cover photo and embedded it again. It shows the new cover photo but the cover photo on the other one didn't change. So it appears that if you want to change the cover photo, you will need to remove the album an insert it again to make the new cover photo appear.
To insert an individual image from a photo album:
From the Insert menu click Images, click Select, then select Photos. Click an album and select the desired image. Then click the Select button. You can select multiple images, See Multiple Images