is it illegal to post a minors picture online?

[bcnu]

Perfectly legal, as long as she's not using it "for purpose of advertising or trade" without your permission. The particular exemption may vary in your state, but that's the law in New York. In fact, she could make it into a poster and enter it in a gallery or sell it as part of a collection, if it has enough creative content to be considered "art". In many states, "art" is exempt under the privacy statutes as "free speech".