Featured image replaces header image

Hello, I have been using Catch Evolution for several years now. For several years it has shown the same Header Image on every post, which is what I want. Recently, for no known reason, it has started replacing the Header Image on every post with the Featured Image for that post. This is very disruptive and unhelpful. It destroys the entire look of the website. How do I get the website to show the same Header Image on every page? Thank you The website, with header image, is http://thechurning.net Two example posts are http://thechurning.net/second-edition-now-available/ and http://thechurning.net/awesomesauce/