Someone recommended "label the back of every picture now."
That is not a good practice, as it can harm the place where the label is, either indenting the letters or causing ink to perhaps bleed through. If you want to label a picture of one or two people, do it in the margin.
For group photos, I use tracing paper and carefully make an outline of the people, so the tracing has all the heads and can be lined up with the photo. Then I label the tracing. I have only one picture of my father, in one of the group photos.
