Changing responsive header behaviour

Hello

I’m using the expound theme. When you decrease the width of the browser (either by resizing the firefox/chrome window or viewing a site on an iphone etc.) the header image at the top has the left and right sides cut off, so you only see say the middle 1/3 of the image.

Is there a way to stop this, and instead have the height automatically resize (thus reduce as the width of the page is reduced) so the aspect ratio of the image stays the same and all of the header image is visible (albeit fit into a width with far fewer pixels)?

Thanks