I was under the impression that you had to have your photo taken at the post office. In the case where you've already provided verifying documentation, having that official also take the picture neatly verifies the picture at the time of the application.
I just looked, and apply by mail is only available in certain situations, one of which is that it must be a renewal, and the prior on must have been within the last 15 years and you must have been over 16. It sort of makes sense with those rules, but still seems somewhat lax to me.
