I think body mikes back then were always on the costumes, usually in the chest area. They were relatively easy to hide in the costumes or to be made to seem part of the costume. In her autobiography, Martin says that even though she was fairly flat-chested until she was 50, she still had to try to flatten her breasts as Peter Pan. So she wore a girdle with the legs cut off and she "pulled it on upside down," over her shoulders. She wrote, "The cut-down girdle was also handy for holding the microphone, because all during the 'I'm Flying' number I was sailing so high into the wings that the sound wouldn't come down unaided. I had a mike which slipped into a little green pocket where the girdle and I met at the top. The batteries for the mike were attached to my harness by a belt at the back, and I used it only during the flying ballet."
It may be that some performers who were body mikes in performance did not wear them at photo sessions.