Link to website in Facebook does not show image

A few people have been linking to my website from Facebook but no images/thumbnails show up. I did a search using the FB debug tool and it shows that the meta property for the og:image file is showing https://wordpress.com/i/blank.jpg Has anyone else had this problem and how can it be fixed? My site is I’m fairly novice so a simple fix or VERY clear instructions would be appreciated.