NCDs IRIS: image or banner behind the title/slogan/nav line?

I have a new site using NCDs IRIS, and would like to place an image behind the top line of the page, which consists of the page title, slogan, and nav links, creating a uniform top-of-page look across all the pages. Anyone know how to do this?

Failing the image option, the nav-padding achieves something akin to what I’d like. How could it be extended across the entire top of the page, from below the title/sloga/nav line to the top?

I’m behind the times for the moment, working with RW5 and Stacks 2…hoping to update toward the end of the year.