When a person's face is detected and tagged in an uploaded photo it can be used is that persons profile photo. Currently we will use the face found on the photo associated with the name. If there is more than one photo then the face found in the "favorite" photo is selected.
Profile Face Images are used in the Tree, Ancestor and Descendant views. Also used in the popup photos when you hover over the camera icon associated with a name in the Family view. It is also used in the Mobile app Names list.
Face detection takes place just after you upload a photo. To detect faces on a previously uploaded photo, click the "edit" link associated with the photo.