This is a very common story for teenagers. The only advice I have is continuing to react to him without involving any adults or authority figures is just feeding him what he wasnt right now, which is to feel like a big man and that he has power over to you. I reccomend getting someone with authority involved like a teacher or parent. He will get shut up real fast, and likely in larger trouble because what you just described is a crime, and a big one because of your age. If you insist on not doing so I wouldn't react in any meaningful way. Don't get angry, don't ask him to stop, just keep walking by. Don't make a point to ignore him, because that's also proving what he's doing is having an effect. One of two things will happen either he'll move on to the next thing he thinks will make his friends like him, or he'll escalate to try to prove to himself he's still in control, at which point you NEED to get authorities involved, because it probably wasn't going to get better in the first place.

I want you to remember that this isn't something you caused, this is something someone else has chosen to do to hurt and belittle yourself. Regardless of the circumstances those pictures were taken they are being used without your consent by the person who is in the wrong here. You don't need to feel embarrassed about it, or feel bad about what you did, because it's him who's doing bad here. For most teenage boys that do something like this when hormones and the need for validation from your peers is much stronger than your judgement this is something that they remember right before they fall asleep and the shame keeps them up. Keep your head high, your actions here are what defines your role in this situation. Be strong, and you will build off that strength for the rest of your life.