Date: 2016oct25 Q. Why is my webpage missing a preview image when posted on social media sites (like Facebook, Slack, Twitter) A. You need this meta tag:

<meta property="og:image" content="http://mysite.com/images/image.jpg" />

The "og" stands for Facebook's "Open Graph". You can test your page here: https://developers.facebook.com/tools/debug/og/object/ Slack uses the system. There are other tags you can use. Slack calls it unfurling so you can google that. http://www.google.com/search?q=slack+unfurling