Realizing that there is some concern with having one's picture taken in the first place; according to those who allow pictures of people to be taken, is there any halachic reason which would prevent one from taking pictures of people in public (ie in a way which breaks no civil law) and sharing those pictures either online or in print without getting the expressed permission of those in the picture?
Rav Moshe Heinemann answered in a shiur,
If you just photograph someone else, and won’t show the picture to others, then I don’t see a way for someone to be makpid – he’s standing there for everyone to see right now. However, if you will show the picture to others, then perhaps the individual would be makpid to not have his picture taken. For instance, someone might be walking on a street which he wouldn’t want his in-laws in a different state to see him walking on. If the person being photographed is in a private location, then you certainly need his permission. Nonetheless, if you entered a building seeing there was a camera taking pictures, then you can’t say you are makpid to not have the pictures available for others since you put yourself into the position to have your picture taken. Therefore, the owner of the camera would be allowed to use your photograph.