Pages - Changing the aspect ratio for set cover images
By default many types of gallery page use square aspect ratio images for the sets view.
To change this, you can update the image profile for the gallery set.
When updating the image profile make sure to change both the set and set @2x versions so that both people with standard and retina displays can see the new image type.
After you are done your site should look like this.
You can try different aspect ratios to give your site a different look: