I have created a BE 2.0 extension which selects the first picture (reading top to down) for showing as a thumbnail when anybody shares the post link on FaceBook. The code is based on Open Graph protocol to select the image and add a meta tag. The open graph
protocol was started by FaceBook and is now followed by many social networkign sites.
You can read more about the extension and download it from this link:
You can see it in action on this BE blog:
Let me know if you need more informatiion.