You'll need to do some pretty fancy stuff when snagging that thumbnail.
That's parsing the page and picking out thumbnails that might want to get used from the tags on the page.
It will need to do this via javascript after the link has been placed. Facebook actually caches their thumbnails for page sharing once a day, so they choose not to go grab it at run time for the client every time. There are certainly libraries (and maybe a jQuery plugin that would let you slurp a URL into memory then traverse it and present some one the fly images.
Check out the Tumblr Share tool. You might be able to reverse engineer from that.
As for Drupal modules this seems unlikely. Would love to hear it though.
You could also think about a third party screen shot service, but that's a pain too.