The cover photo is the one that is shown as a thumbnail of your album. You can add or change your Google Photos album cover from within Google photos, here's how:
This is how you change a cover photo of an album:
- Go to photos.google.com
- Choose "Albums" or go here: https://photos.google.com/albums
- Click the album that you want to change
- Click the photo you want to use as the new album cover
- Click the 3 vertical dots ("More options") in the upper right corner
- Click "Use as album cover"
- That's it.
Source for Customer Service Phone Number of Google Team