Hi Steve,
It looks like you’re using an old version of the theme. I’d try updating to the latest version (1.2.4) here first:
https://www.ads-software.com/themes/sensible-wp/
However, It’s always a good idea to make a backup of your site’s files and database before updating your themes or plugins, just in case something goes wrong.
When using the newer version of the theme, I’m seeing a class of blog-archive-header-bg being added to the header element which you don’t have on your site.
That class sets background-size: cover !important; on the header element, and enables the image to stretch across the width of the site.
Updating will likely make that work automatically, as well as the other fixes mentioned at the bottom of the readme file in the changelog section.
