What you are talking about is the preview image and text that Facebook extracts when you share a link. Facebook uses the Open Graph Protocol to get this data.
Essentially, all you’ll have to do is place these
og:meta tags on the URL that you want to share –
<meta property="og:title" content="The Rock"/> <meta property="og:type" content="movie"/> <meta property="og:url" content="http://www.imdb.com/title/tt0117500/"/> <meta property="og:image" content="http://ia.media-imdb.com/rock.jpg"/> <meta property="og:site_name" content="IMDb"/> <meta property="fb:admins" content="USER_ID"/> <meta property="og:description" content="A group of U.S. Marines, under command of a renegade general, take over Alcatraz and threaten San Francisco Bay with biological weapons."/>
As you can see there are both an image property and a description. When you make changes to your pages
og:meta tags, you can test these changes using the Facebook Debugger. It will tell you if you have made any mistakes (and how to fix them!)