Just because other girls under the age of 15 post risque pictures of themselves online doesn't make it okay. We're talking about children posting suggestive pictures for that people (mainly online pedophiles) can view. This behaviour should not be encouraged.
Right now, it's "just" suggestive pictures. What happens when a pedophile views the pics, sees something he likes, and starts messaging a young girl? You can teach your kids about internet safety but there's no guarantee she'll listen or accidentally let stuff slip (ie. a pic of her in school uniform = they know what school she goes to and can find her).
And third, Miley is the star of a very popular Disney show that LOTS of young, impressionable girls watch. If other tweens see that it's okay to post pictures of yourself like this, they'll start doing it too and like I said previously, this behaviour should not be encouraged. At 14, they're posing in their underwear. By 17, they'll be doing Vanessa-Hudgens-style pictures.
Btw, I'm not attacking you or anything, but I just feel really strongly about this issue. Just because everyone else is doing it doesn't make it right, and there are dangers involved with online predators and I cannot believe that an entire generation is already acting provocative before they hit 16. They're not even close to being adults yet and they're taking pics that present themselves as sexual objects and people are condoning it.