How to change header image for Posts?

My wordpress is hosted by 1&1, and is a click-and-build, so it set up WP (5.6) for me. I selected the Refresh Blog template. The problem I have is, after I changed the header image via Customize, it doesn’t use that image on the Post page. It uses a stock image. And, I dug through the Customize UI, I can’t find anywhere to change that image. If you navigate to a specific post, you can see what I am talking about – some stock image of 2 people hugging while falling through the air…

The page I need help with: [log in to see the link]