How to set featured image for homepage link to FB

Hi, using WP4.0.1 and 2014 theme. I’ve searched the support forums, and find lots of questions on this subject, but still can’t find an answer. How do I set an image to show up as the “link” image, featured image, picture that appears in Facebook, when I post a link to my main home page, front page, main URL, not to a specific post. My blog is https://www.notdunroamin.com Thanks in advance!