I would have her watch a couple documentary about people giving their testimony as victims of revenge porn and how much bullying they got and how much it affected them. I would also let her know that possessing sexual (I know topless isn’t sexual) content of other minors isn’t legal and that she could get in serious trouble for it.
I would then have a long talk about porn and the sexualization of women and how porn isn’t representative or how women have sex and that some boys will try and have her emulate that but that it’s not realistic and talk for a while about consent and what consent is and what consent isn’t.
And then I would do an experiment with her about how easily one can be deceived by the internet and how pictures stay on the internet forever.
I had to do that with my youngest brother. I purposefully pretended to be someone I was not for a day, reached out, made him (easily give me personal information) even used videos and things on platforms he would think people couldn’t pretend to be someone else (Snapchat etc…) and had him send me a silly (non sexual at all of course) picture.
I then confronted him and showed him how much information I had gathered about him, how easily I could track him down, and how now I owned a picture of him that I could easily share on the internet or with his school. I then showed him how easily it is to pretend you are someone else. How easy it is to keep pictures from snapchat even without you noticing (despite the supposed notification feature). And that there is never any way to know who you are talking to online even if you think it’s your best friend, because anyone can hack an account and pretend to be the person and anyone can recover those pictures by hacking an account.
I then made him share a random image on an image sharing platform and gave him the task to get it off the internet (I purposefully shared the image too, so that he would see that once the image travel and is shared, especially across platforms, it’s impossible) and he wasn’t able to.
It really made a mark on him and now he has zero pictures on social media, his accounts don’t have his real name and he has learned to access the functions of the app and enjoy them without exposing himself. He is 14 (was a bit under 12 at the time of the experiment.)
If it was you I wouldn’t necessarily take the phone away but I would swap it indefinitely for a brick phone with no camera and no access to social media apps (which you don’t need at 12 anyway). So she would still be able to text and call but will have no room to take pictures of herself, let alone share them as she has has shown she can’t be trusted with that.