This has been Snap.com 's competitive advantage for years. I suppose Google gave them enough of a head start for them to make something of it if they were going to. Theirs is certainly more mature though. And they have a JS include you can place on your site to enable this sort of image preview on all external links.

Theirs customizes the preview type depending on the content. For example, links to stock information shows a little quote and graph preview.