Name The Faces Now While Somebody Still Knows Them
An unidentified photograph has a short window of usefulness, and the window is the lifetime of the last person who recognises the face.
Every family has a box of people nobody can name. They were obviously important enough to photograph and important enough to keep, and now they are decorative. The identification does not fade gradually across a family, it disappears in a single step, when the last person who was in the room stops being able to tell you. Before that point the task is a pleasant afternoon. After it, the task is impossible, and no amount of effort or expense will bring the names back. It is the one task in this whole subject with a genuine deadline attached, and the deadline is not published anywhere.
Do the naming as a shared activity rather than a chore. Spread the photographs out, invite the people most likely to know, and record the conversation with their agreement while you go. The talk around a photograph is usually worth more than the caption it produces, because people explain relationships, rivalries and circumstances as they identify faces. Work in small batches so nobody is exhausted, and start with the oldest and least familiar images while attention is fresh. Two short sittings a fortnight apart will get you further than one long session that leaves everybody flat and the last tray unopened.
Capture uncertainty honestly. Write that somebody is probably a particular aunt, or that two people disagreed about which brother is on the left, rather than choosing the confident version. A false name repeated for thirty years hardens into fact and is much harder to unpick than a recorded doubt. Note who identified the person and roughly when, because a future reader will want to know whether the caption came from somebody who was present or from a guess made across a kitchen table long afterwards. Recording the source of a name costs you six words and saves the next person an argument they cannot settle.