Add meta tag for og:image

[HereHaveTheseFlowers]

<meta property="og:image" content="thumbnail.jpg">
<meta property="og:image:width" content="1200"/>
<meta property="og:image:height" content="630"/>

The minimum requirement is 200 by 200 pixels.
Recommended size is 1200 × 630 pixels to display a large rectangle stacked under the post text.