Thumbnail when sharing page to Facebook

Not sure if this is a theme question or not... whenever I share my page, the little thumbnail that's generated is the HEADER of the site as opposed to the post thumbnail. If we take this page as an example: http://www.ktcdigital.com/beta2/?p=36 I'd expect to see the Summer Slam image as the featured image when I share the post on FB. However, I'm getting the header image. Any thoughts?