Change default image

I’m using the Supreme Directory theme and am having a hard time changing the default image on the Listings Page / Add a Listing page / etc. It’s the giant image of a map (or sandwich) and I’d really like to use an image with a much smaller height. I’ve been unable to find any documentation that addresses this specifically. I thought this (Set the Hero/Parallax Featured Image for all Listing Pages) might change it, but it did not actually change to my featured image.