Hi there and thanks for your letter.
It sounds like this situation is causing you a lot of worry and you shouldn't have to deal with this alone. It can be upsetting when people take pictures without your permission, especially when you feel that it'll be used to spread unnecessary rumours.
You could try and to talk to your ex and explain how unhappy this is making you feel and see if they will agree to delete the picture. If they won't and carry on to show people then this could be seen as bullying.
You have the right to tell a teacher or another trusted adult about this. Most schools have an anti bullying policy and should take action against situations like this.
If other people do see the picture then it does not have to mean that they will think badly of you. Pictures don't always show what really happened and you could tell people your side of the story.
Sometimes when we worry about something it can seem like everyone else is thinking the same thing. You might be surprised at how understanding some people can be or how quickly they forget and move on.
It might help to talk this other with your friends, they could have some ideas about helping you deal with this.
You're are also welcome to post a message on the ChildLine message board to ask other young people for advice.
Hope this has been of some help
Sam