Facebook or any other social sharing platforms generally pick up an image from shared web page
using their own algorithm. But there is a way that you can point them to an image that you want to be shown as a thumbnail.
The solution is to add an open graph meta tag in your web page with the link to the desired image. The open graph standard is followed by many social sharing sites including Facebook. Here is an extension to add the meta tag on its own to your BlogEngine's posts.