If she posts a picture of a hairstyle she is considering, it might be appropriate to say it looks like a bull (something).
If she already has the haircut, then you compliment it or say nothing at all.
If she posted it to your wall, to solicit your opinion specifically, then you say something truthful and nonjudgmental. Like, "It really frames your face," or "I really notice the highlights now," or "It gives your hair a lot of body/volume/whatever." No mention of whether you liked it or not.