EDIT:
Your site doesn't share correctly.
I went to share this on LinkedIn:
* What Slack might learn from its Open Source alternative | Mattermost || https://www.mattermost.org/what-slack-might-learn-from-its-o...
And I see this:
http://imgur.com/a/evVZF
You're using WordPress so you can check out:
* Yoast SEO — WordPress Plugins || https://wordpress.org/plugins/wordpress-seo/
This will add the right meta tags to make the thumbnail and image and description show up through shares with minimal effort on the part of the content creator.