Apple’s Photos app is remarkably good at identifying people in your snapshots and collecting all the pictures that contain a particular person into a group in its People view. At its top level, the People view shows a thumbnail photo for each person, picking one automatically from all the available photos. Needless to say, it doesn’t always pick the photo you want, so if you dislike what’s there, you can change it easily on the Mac. In Photos, click People in the sidebar and double-click the thumbnail of the person you want to change. If necessary, click Show More to see all their photos, then Control-click the desired photo and choose Make Key Photo from the contextual menu.
