After the introduction and relative success of application rich snippets, Google is now placing the authorship rich snippet with author images being flashed next to the articles credited to them all over the SERPs.
What Google has done, is that it has linked the authorship image to author's Google+ profile. So, whenever a search is made that shows up an article credited to the Google+ user, then the SERPs will fetch the profile picture of that particular user and place it as a thumbnail image on the left hand side of article description.
Though Google has previously mentioned that "you'll need a Google+ profile with a good, recognizable headshot as your profile photo", they haven't forced it yet. It is unclear still if Google will stick to their “headshot” demand or not, since the changes have worked well for some, we can expect Google to come out with a statement soon after all “need” is not a strong enough word. Until they do, you may optimize your Google authorship image to increase your very own CTRs.

