Op you sound like a thoroughly decent and loving parent, and your life time work with teens will surely help you in many areas where we will flounder later!!
It is hard working out the middle ground, and what is imposing 'our world view' on them, and what is appropriate for a child of 13. It is a very fine line that most of us make many mistakes before hitting on something that feels right.
This is going to send very far out, and at the risk of sounding new age I will put it out there. Are you still hugging your teens (when they allow it)? Are you still kissing them goodnight? Are you still connecting them on a physical level as you would have done when they were young?
Some parents back of instinctively as the child becomes a teen, new body boundaries are often put in place by the teen or the parent, and a lack of affection is sometimes the outcome. I say this because I have a theory that many teens of this age and older are somewhat starved of affection, and they go looking for it elsewhere. It may be of their own doing, or it may be that parents feel it is no longer feels right. Every human being needs to be touched, to be held, to feel loved and to feel care for, even snarling 6ft teenagers. We all need to know that we are cared for, and often it is around this time when they in fact need it most. Girls especially will go out and look for this kind of affection from friends, and sometimes boys.
Really being present, listening, developing hobbies together, making sure there is physical connection if the teen is happy to be hugged and cuddled (not otherwise as it becomes an imposition) Really making the time to focus entirely on them even for a short amount of the time daily is really undervalued in this country particularly. There is a value system that once children hit the teens we can check out to some degree, when often the opposite is needed.
This has nothing to do with the way your dd dresses or what she posts, but it may be a stepping stone for the attention she is likely to receive especially from boys, and if she is vulnerable to needing affection she is more likely to get involved. The girls you identify as 'trouble' or often girls that are simply deprived of attention/affection and are taking care of that basic human need themselves.
It all depends on the reasons for her doing this, is she copying others? Does she like the attention? Is she doing it to shock you? Rebelling?
Or simply playing, and dressing up?
Either way the photos need to stop being posted on SM, for her own good, she will thank you in a few years I promise you :)