The first gallery in the post will used how featured image for post and on front page too. But you must add the featured image for your post also – this featured image will be used by widgets “Sagablog: Popular posts”, “Sagablog: Popular posts (slider)”, “Sagablog: Recent posts”, “Sagablog: Recent posts (slider)”, “Sagablog: Related posts”, “Sagablog: Related posts (slider)”.
Here you can see how it look like on site:To use this post format you must choose “Format -> Gallery”:
When you create gallery – you must choose size – “Custom Large 1040×600”:
In the drop-down menu “Link To” you can choose: