Just thinking out loud here... I'm wondering about the difference between the types of pages on which the photo was removed and on which it stayed.
Is your site set up as a blog, which might contain several instances of an article... ie, on a permalink for the article, and also on home, and perhaps by subject and by date?
If so, is there a chance that Google is changing the photo display to appear only on the most prominent copy of the article?
Also, another possibility... had you applied the author tag only to articles themselves, or was it also applied also to the container pages (like home, categories, and chronologically) that contained copies of the articles?
One other possibility... were some of the articles things like, say, news announcements, which strictly speaking aren't original content, and might not deserve an "author" tag?
On which kinds of pages did the photos remain, and on which were the photos removed?