"Each photo you decide to share is opt-in" - I don't think that is really how permissions work on either Android or iOS. Once you have given the app permissions to access your photos, they can do so - with or without the user explicitly choosing which ones (please, someone correct me if I am wrong on this). So Snapchat/Instagram/Facebook might actually be doing stuff with your photos "behind the scenes", and you wouldn't necessarily know it (assuming you granted the "Photos" permission in the first place).