By online I mean on social media, Flikr, on a website etc, not anything that is password protected or that you have to invite people to view such as Google Drive.
Is it for safety reasons? You don't know who might end up seeing the image or what they might do with it?
Is it for anonymity reasons (adopted or LAC for example)?
Is it because they aren't old enough to decide for themselves? At what age would they be old enough?
Is it because they are old enough and have asked you not to?