<img src="your_thumnail_image.png" style="display: none;" />
<meta property="og:image" content="http://img.youtube.com/vi/IYVCXc2OVVM/0.jpg" />
Check this og:image sets the image
https://developers.facebook.com/docs/sharing/webmasters
eg.
Code:<meta property="og:image" content="http://img.youtube.com/vi/IYVCXc2OVVM/0.jpg" />
<meta property="og:url" content="http://www.rls-movies.com/category/releases" />
<meta property="og:type" content="article" />
<meta property="og:title" content="Watch Now" />
<meta property="og:description" content="Hot 2016" />
<meta property="og:image" content="http://www.rls-movies.com/wp-content/uploads/2016/05/bmw-play.jpg" />