The rule
Before you post a photograph with someone else in it, ask them. It takes a moment, and it's a courtesy almost everyone appreciates. They may have reasons you know nothing about for not wanting to appear online.
Why it matters
A picture on the internet is hard to take back. It may show where someone was, who they were with, or simply how they looked at midnight. All of that is theirs to decide, not yours.
How to do it well
- Ask at the time: "Do you mind if I share this?"
- Take it down straight away, and without argument, if someone asks.
- Take extra care with children, and with anyone who might be at a private event.