I also thought about adding existing pictures, but for me it looks that the other way round it gets more attractive: take the pictures via ActivityDiary and they will automatically be available on your phones gallery... Adding existing pictures later may be only worth while editing the history.

On the other side some users might consider it more convenient to take the pictures with their usual photographers app. For that use case an "auto grab all new taken pictures and attach them to the current diary activity" might be interesting.

Attaching audio notes could be also a good benefit.

Can you create a github ticket (or better two) for those ideas?