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How provocative are the pictures? I don't know anyone who posts nude photos or half-naked pictures on Instagram. I know a lot of women post selfies on there but the ones I've seen don't post anything too revealing. Maybe your boyfriend's female friends are different, I've just never seen women post pictures like that on Instagram.
But anyway, if it bothers you, you should definitely tell him and ask him why he follows them if all they do is post provocative pictures. I can understand how that would upset you. I do think you should remember though that just because he may follow these girls on Instagram doesn't mean he sees everything they post or follows them JUST to see these types of pictures. My point is, perhaps it's really not a big deal to him so it may be difficult for him to understand where you're coming from.
So, I think you should just calmly approach him about the topic, don't come at him in an accusatory way because that will just make him get defensive. Just talk to him in a mature way and hopefully he will understand and you can come to some sort of compromise or agreement.