It's a shame when someone circulates a picture or outs a person without their consent. They probably think it's funny but they don't realize the potential harm it does to that person.
I suppose the only option left for you now is to speak with your HR person and see what type of damage control can be implemented. I realize you may not have been ready to transition at work but it looks like your co-worker has let the cat out of the bag. It will be hard of course but with help you can do it. Who knows, maybe it will be a good thing in the long run but that doesn't matter, the harm has been done now.
I would at least confront the person who did the posting and ask them why they did it and how would they feel if you had posted any embarrassing pictures of them for all to see. Maybe that will make them see what they have done?
Love,
Clare