I am fascinated by "found photographs." These are photographs that have become separated from their context - they are anonymous, orphaned. We don't know who the people are in them, and often we don't know where they were taken. They can of course be mined for clues, but we very rarely find out everything about them. They are therefore typical archaeological artefacts. Each records a moment in time, but we have to carefully examine and research it if we are to learn anything about that moment.
Found photographs can be sourced from junk shops, antique shops, charity shops, rubbish skips and sometimes from the roadside. I've only just begun to study them, and this is just the beginning of my sharing of what I stumble acropss.