Is the concern the doctor, or someone in their office misusing the pictures? Or is it the fear of having such an image on your phone or sent over the internet at all? Or is the concern that the very concept of such an image being recorded is morally wrong? Or is it something else?

I think the most reasonable concern would be having the image sent over the internet. I wouldn't be afraid of anybody in their office misusing the picture because they're trained and it's just another day in the office for them. Body parts are body parts, it shouldn't be morally wrong because of where the photo is, I think that's problematic thinking in my opinion.