First of all you need to consider that this is a responsive theme. So what will a long full width header image look like on a iphone, ipad, small desktop, large desktop… etc? It would be hard to make it look good imho.
To put something at the top like this you would need a custom header.php file in a child theme.
The preferred place for a large header image would be at the top of the main content area. This way, the responsive part of the theme is less affected.
There is a hook for this, so that in a functions.php in a child theme you could do something like: