The beauty, the sexiness and even eroticism is in the eyes of beholder far more than any objective value. I, for example, speaking about porn I use, have quite a fetish for nude female form (even while I position myself as nudism advocate claiming nudity's no big deal), but have no interest in explicit genital shots (or most plain sex action), for that matter. Further, I adore small breasts, and find most artificial "enhancements" ugly, including tattoos. Seeing how much of that is around, I must be weird.
So, her first problem is about guessing tastes of the receiver(s). The second is purpose of communication, context and intended use of the picture(s). Sure, she doesn't have much control over the later, and I advise to consider that (and she may not, but that's whole different discussion).
Without splitting hair between "sexiness" and "eroticism" and with regard to my tastes, implied nudity is better than explicit. If there's any clothing at all I would prefer lose clothing that seems about to fall off or just casually thrown on temporary implying there's nothing under, even to elaborate underwear that mostly doesn't do it for me, but the exact opposite can be true for others.
Attitude is what matters most. And, natural look, comfortable, unmistakable self not a made up artificial artwork, although, depending on how well it's done, that have it's places, especially in circles appreciating cosplay. Also, if it makes her appear younger, it can't go wrong (unless she's still a teenager, and even then only if she overdo it badly).
Long hair is very sexy, and great for playing in a picture. She may, for example, obscure most of the form of her breast, allowing hint of a hard nipple to peek try. Gorgeous hair also naturally lends for half turn back shots (the over-the-shoulder look at the recipient). Such a shot can demonstrate full nudity without revealing anything at all (except the inciting smile and glint in her eyes of course), and I believe would be considered tasty in most contexts.