When a link is shared on Facebook, Facebook uses one of the images on the shared link page as a preview image.
Here is how you can control the image that is used as the preview thumbnail.
You need to set the meta tags title and description and in addition add link tag with the rel attribute set to “image_src”.
<meta name="title" content="title" />
<meta name="description" content="description " />
<link rel="image_src" href="thumbnail_image" / >
For more information, see: Facebook Share Tags
And here is page that you can test by sharing in Facebook, to see how it works: http://www.aggregatedintelligence.com/Samples/MySocialPage/twitterhome.htm