Change a Photo Album’s Cover Image
When your organize photos into albums you might not always like the photo it shows as the default. The main cover photo is called the “key photo” and it’s easy to change.
Simply navigate to the photo in the album, long press, and choose “Make Key Photo.”
You can also set key photos for people. Under Albums > People & Places > People, select a person, then press and hold on a photo, and select “Make Key Photo.”