Google Photos Album

In a Google Photos album click the share button and copy the link. This one is

Use the embed URL function in the Insert menu to insert it into your site.

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

This time I got the shared image URL and inserted it using the Embed URL function.