This will show you how to set the Facebook thumbnail for shared page content from your site.
you will need to add in the following meta tag
<meta property="og:image" content="https://yoursite.com/image.png"/>facebook will then use the appropriate image.
there are plenty of open graph meta tags to use
see this page on facebook for more
alternatively you can use the following meta tag:
<link rel="image_src" href="https://yoursite.com/image.png"/>