At Pubcon Las Vegas in October, Google's Matt Cutts commented in his keynote that they would be tightening up authorship in order to combat spam. Google found that if they removed about 15 percent of the lesser quality authors, it dramatically increased the presence of higher quality authors within their search results.
In mid-December, Google rolled out new authorship algorithms designed to display authorship for what they consider relevant and interesting authors, Google has confirmed to Search Engine Watch:
We made some minor updates. We had been showing author information whenever we could based on authorship markup, email verification, and other signals of authorship on the web. In mid-December, we rolled out new algorithms designed to show author photos when they're more likely to be relevant and interesting. (For example, the algorithms now try to estimate the quality of documents an author typically writes).
Authorship is also going to put a greater emphasis on high quality content, so that authors that write higher-quality content are more likely to have their authorship with image displayed. Said Google:
If an author typically writes high quality content, that author is more likely to be relevant to you.
While quality content is important for SEO and search result rankings, it isn't surprising that Google is using this as a signal for when to display authorship, and to display authorship with or without pictures.
Google+ will also play a large role when those authorship photos are displayed. If you have someone in your circles in Google+, their social signals will increase the likelihood that you will see that person's image in the search results. This will make it even more important to authors that they are being circled and promoting themselves on Google+, particularly since search results with authorship displayed (especially from those authors the searcher recognizes) tend to get higher click-throughs. According to Google:
We also rely on social signals designed to show you author portraits for the people you've circled on Google+. So now you'll see 20-40% fewer author photos overall to start with, and even when you don't see an author portrait you'll still see the author name.
Authors began noticing the change around December 13-14 when their names were displayed, but their images were not. Nothing in an author's Google+ account, or with authorship specifically, lets an author control whether the picture displays. The algorithm definitely favors higher quality content, as well as who the searcher has in their Google+ Circles, when deciding whether to display an author's image.
Google also told SEW:
We continue recommending that authors properly implement authorship to help users discover their content more easily in Search and other Google products. Continue focusing on creating great content that people want to read and share and the rest will follow.
Regardless of whether authorship is displayed, Google is still showing Circle counts for both. But the overall number of times an author has been circled doesn't seem to have an impact for those who aren't in the searcher's circles.
There is a wide range in circle count and displayed authorship. However, circles will still impact authorship for the authors the searcher has circled.
Whether an author's portrait is displayed seems to be not only influenced by personal quality, but also the quality of the site that the author posted to. The same author can have his portrait shown for one site, but merely a byline on a second site.
"It also seems that Google may be refining their assessment to sites and even individual pages, and not just white-listing authors and granting rich listings on every single thing they write," according to Dr. Peter J. Meyers (aka Dr. Pete) from Moz.
Meyers also noticed the authorship drop. On November 24, 2013 they saw peak in authorship of 23.71 percent. By December 19, 2013, it hit a low of 20.03 percent, which is a relative drop of 15.5 percent.
This relative drop is definitely in line with what Cutts said at Pubcon. Unfortunately, he doesn't have data between authorship without portraits and authorship with portraits.
An interesting point he does make is that authorship actually increased slightly after the Cutts keynote before we saw the relatively steady decline, which tracks correctly for when the authorship algorithm changed in mid-December.
Google has also become more strict with the conditions for when they will display authorship. If you are using authorship for a WordPress-based site and use Yoast's SEO for WordPress plug-in, the default setting is turned on for posts, pages, and media, but Google recommends that you use rel=author on posts only.
Until this is updated, you will want to ensure that you only have rel=author for posts checked. If you use WordPress and saw authorship removed completely, it could be the issue.
Authorship will continue to be an important aspect of Google search results, and it makes a lot of sense for authors to ensure they are only promoting themselves with high quality content and promoting themselves well for Google+. While having your image displayed is ideal, there is nothing beyond what's already been mentioned in this article that an author can do to ensure that picture is displayed.