If this happens when using the "Add a Link" feature, you are missing one tag on your page. You need to include the og:image tag - not just for us, but for any sharing. Without that, the "shared to" platform (Facebook, Twitter, LinkedIn, etc.) won't know what to show with the story.
Here's an example:
<meta property=”og:image” content=”http://www.yourdomain.com/image-name.jpg” />
Here's more background on the tag:
This is the most interesting open graph tag for marketers because a picture always helps content stand out. This is how you ensure that a particular thumbnail will show when your page is shared. It can be very helpful for your conversion rates.
Make sure you set the og:image you choose, otherwise Facebook will show unwanted things such as an ad banner scraped from the page, or nothing at all (as seen in the image below). We definitely don’t want this:
It’s important to remember that if your page is static and you don’t use any sort of content management system (CMS) (e.g. WordPress), you need to change the og:image manually for each of your pages. If your website is controlled with a CMS and you installed the relevant plugin, the og:image tags are assigned automatically for each page. Look for the list of plugins further down.