That was a cover done by Hipgnosis, a work by Storm Thorgerson.
The Naked "girls" you mention are actually two kids, a boy and a girl. The cover is a composite of more than 30 photographs taken over a 10 day period in the rain at a castle that was chosen as the "ideal spot" before the weather turned bad and forced a black and white photo session rather than color.
At one point, I was told by a person close to Robert Plant that the little girl in the photo is Robert Plants own child, Carmen Jane.
