I retained the original answer with [updates] to help understand some of the background to the final decision made by Google.
Unless there is something new, [Update: and apparently there is. Author images and circle counts have disappeared on June 25th 2014.] Google reduced the Author images after doing an audit a short while back. The reason mentioned was that CTR was suffering and the Author images were no longer doing what was intended- creating strong value brands.
[Update: Google was vague, but they claim that CTR did not change and that cleaning up the SERPs was a priority. Others disagree with the CTR claim and it contravenes earlier claims by Google.]
With so many authors click through rates began to drop for SERP links with Author images which is contrary to the purpose of creating author branding in the first place. The intent of the Author image was to identify strong author brands, but that had become diluted and the SERPs contained too many Author images of lower quality authors. Google audited the pool of authors and disabled images in the SERPs for the lower %15 performers. It may be that they broadened the scope beyond the %15.
They divided to pool to three categories; full author details including image, author details without image, and no author details at all. It is possible that even author details without image will be divided. So what is left is a set of stepping stones that an author has to progress through to get an image besides their SERP links. Think of this as AuthorRank for a lack of a better term.
Sorry folks but it was inevitable. With everything, there is a swing of the pendulum. With everyone seeking Author images, there had to be a cutback at some point. [It appears that Google did not want to deal with spam.]