For the record, in advance, I have five girls. While I can't fully assess their feelings about their changing bodies, the older two seem to be navigating puberty smoothly. Here's what we do, in the hopes that it's helpful to others.
While we all know that images are airbrushed, it's hard to keep in mind when it's done constantly and so well. I've told the girls and shown them a few videos about the process, and talked about how airbrushing (and auto tuning for music) became industry norms and thus unavoidable. We don't have a TV, and we watch movies on the computer a few hours per week. My oldest had an instagram account which I had her close because her feed was filled with scantily clad girls "doing their hair" in lingerie. She has an old phone and no facebook account. Having almost all her waking hours filled with real people and real interests is probably the most important factor.
Girls need to know that those people with "amazing" bodies in real life have a combination of luck, hard work, strict eating, and flattering clothing. Without making a moral issue of it, I think it's sufficient to say that not everyone wants to spend the time and energy to do this, and there are arguably more meaningful pursuits.
On the other hand, if your daughter is actually over or underweight (as assessed by body fat rather than BMI), acknowledge the truth when she brings it up. Offer to support her in getting bloodwork to check for metabolic defects, and changing her habits,
When I used to complain to my mom about being a little heavy (10 pounds) in high school, she told me that I was welcome to exercise, and there was nothing wrong with my body, I just needed to move more. And that I was attractive anyway, and I was blowing my extra bit of weight out of proportion in my teen angst.
All of which was completely true. I had a sweet tooth, I liked to read more than exercise, and it really wasn't much weight. In fact, it might have been a normal family pattern for my age. It was good to be told the truth, even if technically she could have been more supportive.
While families have different guidelines for modesty, the principle I've used with my girls is that too much skin distracts others from seeing your personality. While it's sometimes debatable how much skin is too much, when we go to amusement parks and the fair, it's pretty clear which outfits show enough skin that you notice the body more than the person.
Help them shop.
Unless it's a gift-giving holiday or they have a definite need for an item, they buy it for themselves. This in itself limits the amount of shopping to every couple of months. The teenagers might want more than one pair of jeans at a time, so they buy themselves another pair or two. (The younger girls get clothes from us at Christmas, Easter, and birthdays; if they buy themselves clothes in between, it's with money earned from extra chores or good behavior prizes.)
Shopping with them allows me to talk about color and cut, and hold boundaries about taste. Since it's their money, I will allow them to buy something that is objectively unflattering, but I'll tell them I think the color or style is wrong for them. Generally they put it back. Years ago, when it was my money, they weren't so careful and resented me making style decisions "for them".
Watch Your Language.
The words you use are the words she'll use. So model gratitude, because having a body and being alive is a blessing in itself. If there are things you like about your body, say so, and you might clarify whether it's just luck or habits. If there's something you don't like, you might mention it to add perspective.
I'm naturally knock-kneed and it was embarrassing to me, until I figured out that 1) no one cared as much as I did, 2) it wasn't like the rest of me looked like a model, and 3) my legs straightened out when I strengthened my core, which also made the rest of me look better. I've shared this with my daughters when the topic came up, and also how many things I like about my body and how I'm used to all the quirks.