But I still are having issues with your first message to girls. I am the father of two young girls myself, so this hits close to home.
First things first.
1) For one, it seems that you are not wearing a bra.
Must all girls who post photographs of themselves online be wearing a bra? How is it that you're so fixated on a girl's breasts that you *discover she isn't wearing a bra*? At what age should we begin requiring girls to wear bras? Or how large should their breasts be before wearing a bra is demanded?
Do you see what I am getting at here? The entire foundation of your suggestion is dependant on the subjective interpretation of pictures. And I find the breast-fixation especially problematic. But more on that later.
2) Those posts don't reflect who you are! We believe you are wonderful and interesting, and generally quite intelligent. But, we had to cringe and wonder what you were attempting to do? Who are you trying to reach? What are you attempting to say?
Why do you care? Is she posting pictures of herself taking drugs? If so, then by all means, sound the alarm! But you understand what I believe she is doing? I think she is figuring out her body. And good for her. Because she's happy enough with her body to put photographs of herself online. The fact that the photograph is taken in the bedroom is unimportant.

And it's great that you believe she's lovely and interesting. But have you told her that? Have any of her friends told her that?
click what now would be a great time! Our media are obsessed in presenting unrealistic representations of women, and these young girls are being continuously bombarded by hypersexualized caricatures of themselves. You know who gets labeled as wonderful and fascinating by the media? Miley Cyrus. That is who. So instead of blocking your son's friend's graphics, why don't you have your son tell her she's lovely and intriguing?
3) I understand your family would not be thrilled at the idea of my teenage boys seeing you exclusively in your towel. Did you know that once a man finds you in a state of undress, he can't ever un-see it? You do not want the Hall boys to just think of you in this sexual manner, do you?
And how just is it the girl's fault that your sons see her in a sexual way?
You understand what's fascinating about being human? I mean, 40 was really old. This is why individuals are effective at reproducing as early as 12 or 13 years old. Because that's when individuals had to start reproducing if they were going to live long enough to ensure their children survived to maturity.
Fast forward to modern times, and guess what? Kids still start to develop sexually at about the same age as they did hundreds of years ago. But what's different is that we've developed all of these rules and expectations about how children can *express* that sexuality. I am not in any way recommending that we should let kids start having sex that early. But what I *am* saying is that it is completely unrealistic to expect our kids to totally dismiss their bodies as they're starting to change.
Also have you taught your sons the difference between a state of undress and sex? They are two things, you know. Nudity is not a condition for sex, and sex isn't a prerequisite for nudity. God created us all naked. Clothing is our own human innovation. God did not mean us to feel embarrassed while we're in a state of undress, even if we are in the existence of others.
(They're closely related both involve the practice of non-sexual nudity. Your research will allow you to understand the little differences between them both.)
Again, I Will bring out the swimsuit argument here. Listen, I was a teenage boy once. Believe me, it takes HARDLY ANY help to visualize a girl naked in her bedroom. If they've seen their female friends in swimsuits, on-line photographs are not going to make much difference.
To conclude, I think there are a couple of lessons here:
First, educate your sons that women are their equals in every way.
If you can not say something nice, do not say anything at all.
As an adult, I 've lots of friends on Facebook who I don't always agree with. But I do not call them names, and I don't block them. People express themselves in different ways.
We should teach our kids these matters because most individuals who are sexually assaulted are attacked by someone they know either a friend or a relative.