Is it illegal to take a picture of someone without them knowing?

[Kimberley]

My friend took a photo of me without me knowing then posted it on a social website without my permission. She posted it on snapchat btw. I'm worried because she sent it to people that I hate, boys, crushes etc. Now they have all seen it and they can screenshot it and show it to friends. I hated it and told her to take it off and she won't. I didn't want it on any social media, I don't want her to have that photo of me on her phone to show others, I don't want people to see, I don't want it to be on a social website and I didn't want or ask her for the picture to be taken. I said to her its illegal but I don't think she's convinced enough to take it off unless I have proof that its illegal. Thankyou if you reply