you’re 99% of the way there 🙂
Edit the Header Element and on the Site Header tab check Navigation Colors. This will enable background and text colors – just change the Background to a transparent color.
Edit the Header Element > Background Image – and set this to Featured.
Then check Disable Featured Image to remove it from the page content.
Make sure the Header Element is set to Full Width.
And now give is some Top and Bottom padding – i would suggest 23% top and 23% bottom – ( the total 46% is equal to your images aspect ratio so should display the full image now ).