This may be Google's way of compelling site owners to join Google Plus. I have not yet seen any compelling evidence of this, but I suspect that you should join several Google circles to improve the chances for having your image show up. I notice that quite a number of Plus images that show up in search results have a notation similar to, "in 1,254 Google+ circles."
Matt Cutts recently stated that they are currently not looking at any social networking signals, but I suspect that they are or will soon be looking at their own social network.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin