Trouble with display image for "Structure" theme

I recently installed the Structure theme for my blog, No More Popcorn. One of the features of the theme includes displaying the set display photo for the most recent blog post at the top of the page. For the last two posts I have done, it has not shown a display photo, despite the fact that I’ve set one. How can I fix this, and make sure that it doesn’t happen in the future? At present, my front page looks pretty dumb without a featured display photo, and I’d really like to get that taken care of.