In order to help you Tag faces on previously uploaded photos, we have added a new sort category "Face Detection Candidates" in the Photo page of each Album. You need to be signed in as Administrator to use this feature.
 Log in as Administrator
 Pick "Photos" under "Media" in the menu.
 Click on an Album.
 Choose "Face Detection Candidates" in the sort selection.
 Click "edit" on any photos photos that have the Tag icon.