The person who dislikes being photographed still deserves to belong in the event.

Not wanting to be photographed is common and rarely about vanity alone. Someone may dislike how they look at the moment, or be tired, or simply not want an image of today existing anywhere. The trouble is that avoiding cameras gets read as being aloof, and after a few years that person is genuinely missing from the record of a family.

Respect the no without arguing, and stop turning the lens on them for a laugh. Find other ways to include them: their handwriting, their cooking, a photograph they took. Ask again occasionally and gently, in case the answer has changed.