In custom page and article, the "Custom Meta Header" is used to add meta tag to the page. For example

<meta property="og:image" content="http://example.com/v2.jpg" /> 
<meta name="description" content="Some descriptions" />
<meta name="keywords" content="meta tags, meta description, meta keywords for search engine optimization /">

So when you share the link at facebook etc, they will use that as preview image. 
You can use https://developers.facebook.com/tools/debug to see 
how the preview is going to look like on facebook.