Getting back to the Gates mug shot being used as the generic profile picture in Outlook 2010, Ars discovered that the silhouettes in the two photos are a perfect match. No confirmation from Microsoft has ever been offered on this, despite the obvious evidence that seems to indicate this is the case.
Since then, Microsoft has switched to other generic photos for its apps, so Outlook 2010 was the only version that featured Bill Gates’ mug shot for the People Pane. In Windows 10, for instance, the Mail app uses a generic circle that shows the sender’s initials, instead of a silhouette created using some employee’s photo.
But I certainly wouldn't put it beyond some company employee to do such a thing as a running joke (with or without approval).
Given that the source is already dated, I wonder if evidence for or against this claim exists by now.
Edit: As far as I can tell, the only source to the story is the above cited ARStechnica article by Peter Bright and his (?) image comparison. I haven't found any independent source or validation so far.