How well do you know this girl? If she rides at your barn then I would have a discussion with her, maybe with the barn owner present. Tell her that it's radically inappropriate, and that if she thinks you are treating your horse inappropriately then she should come and tell YOU, not try to receive validation from the internet.
I haven't ever seen any pictures of your riding, but there are folks out there that shouldn't be doing what they are doing with their horses- jumping too high for their skill level, riding incorrectly, etc. I've seen some pictures that made my jaw drop. I cannot comment on whether or not you're doing that, but it's absolutely inappropriate for someone to post a picture of a kid's riding (especially someone that she knows) for all of her friends to ridicule. ESPECIALLY since she's 25 and you're 16. She should know better- EVEN if she radically disagrees with your riding.
If you know any of the people that are commenting on the post, then I would contact them privately and explain the situation. I'd also ask your friend if she would avoid posting any less-than-flattering photos of you and your pony until this has been worked out. It shouldn't have to be that way- years ago I took a video of a friend of mine's pony refusing a jump. She lost her position, and it was pretty funny to watch. She went around to the jump again and looked perfect. Even years later we like watching and laughing at that video, but I'm sure she would be absolutely mortified if someone took a still shot from it and started picking apart her riding and said she was abusing her pony.
The damage has already been done, but one possible way to prevent it from happening again and get the picture taken down is if by posting it as her work on a public website, she was using someone else's photo without permission. If your friend makes a stink about that then it may encourage her to take the photo down, and make her think twice about posting something like that again.