How awful for you (hugs).
My DD has had Instagram since she was 10. Her account is private and I have my own one too and of course I follow her. I set it up, know her password and always double check who she allows to follow her (they have to ask before being accepted). Anything I am not happy with I delete or if if she doesn't know them (not matter what she says), I don't accept them.
I have seen some questionable things on some other children's Instagram, although to be fair it is more quotes than inappropriate pictures. That said one of her classmates occasionally posts pics of him chest upwards and I don't think this is acceptable for an 11 yr old boy; thankfully my DD just laughs and says well it is obvious his mum isn't checking his account. She is more interested in arts and crafts, searching for nail art accounts and Minions than posting selfies. She does not post that many pics of herself like her friends but I am lucky in that she knows all the dangers and is always careful. I am well aware it might not always be that way and at some point I know she won't want me to have access. I like to think that she will always be happy for me to see her social media accounts but who knows in a few years? Our children can start off sensible then go down the secretive route!
There was a child in a school that was in Year 4 and because her mother is, how shall I say, a tad flighty with the young me and had many boyfriends and was always posing saucy selfies, this girl thought I think it was normal. I have no idea if her mum knew or cared but I know other parents removed her from their child's social media. It was quite sad really as I think this girl just was copying her Mum and thinking it was okay. Quite sad really. Such a young age to be doing that.
I think you did the right thing removing it. I also agree with cariadlet about saying she would have no control over pics that could get plastered everywhere and embarrass her. That is a good start on telling her the reasons why it isn't right.
As a teenager that would prove utmost embarrassment and distress so perhaps this is the way to dissuade her. She could have just been testing the waters especially if her friends are doing same. It is disturbing to think our children at some point will be posting the kind of pics we would usually reserve for the over 18s.
I hope you can get this sorted x