This is a noticing exercise disguised as a writing exercise. The student practises shifting attention from the foreground subject to someone the photographer was not focused on. Backstory writing trains specificity. A name does more work than a description, an action more than an adjective. Some students will pick a foreground figure because that is who the photo points at. Send them back. The activity only works if they choose someone overlooked. The final question is the curriculum content. Did they make this person matter?