I have teen girls. 
We had a rule back when we first started out with the phones and SM, no photos of themselves are to be posted full stop. They can post pictures of cats, dogs, sunsets, slime and all the rest, but the rule number one is that you do not compromise yourself (and future online)
In fact, my dds were keen not to do this when we discussed how the photos can be used, who could be looking at them (middle aged men pretending to be girls) and what was today's great idea, is often tomorrow's total cringe. I showed them photos of my teen years, dreadful outfits and awful hair, and mean girl faces and asked them if I had posted them on line would this now be a good look? It was empathetic no.
So we used the grandparent/headteacher rule with every single thing we post, if they would not like it, it does not get posted, that is the case for everything inc messages.
The girls self police now, they are incredibly protective over their own privacy. The problem is if your dd is posting any kind of photo even among her own circle it can still end up being used by anyone.
I check their phones a lot, and I check what is happening on all SM sites often so I am aware of what my children are doing. It is an agreement we have if they are to use their phones.
Educating her about the use of her photos, what it will do to her job prospects and what her teachers will think when they see it in a gentle and open way. Ask her questions about it. Send her a photo of you dressed up in a crop top and short skirt and ask what message she gets from that photo? Delete afterwards!!
Also introducing other role models to her, stepping up her sports and activities so she has less time to get made up and dress up, and generally being very conscious that it is most likely a phase anyway, and she is just working out who she is.
If you think of it in context of 'playing' with her image a little like a doll, rather than anything sexual, and I am certain she is just trying out new things - then it changes the way you see the photos and her actions. She is still just a child. That is not to say you allow her to continue to post, why not compromise that she can take the photos, but she keeps them stored in a file on her phone and doesn't post them.
It is the posting of the photos rather than the photos themselves I should think are causing you the most worry, so put some boundaries around the posting is what I would do.