The person declining a photograph should not have to explain why being absent feels better.

Stepping out of a photograph draws more attention than staying in it, which is why the request for a reason is such a hard tax. The reasons are usually private: how someone feels about their face today, an illness, a body that has changed, a wish not to appear online. None of that is owed to a room holding a camera.

When someone steps aside, let them, and move the group along without comment. Do not tag or post anyone who has declined. If you are the one declining, you are allowed to say no thanks and nothing else.