Selected answers from the Dumb SEO Questions Facebook & G+ community.
Masatake Wasa: By Open Graph (og:image) - further information at http://ogp.me/
Collin Davis: Facebook has open graph whereby you can specify the image that you want featured in the snippets when people share the page. Have a look at the source code of a website like searchengineland who have implemented this correctly.
Daniel Zammit: Try some Facebook markup - this link should help - http://bit.ly/1cUscYC