The binder is gelatine. As humidity grows, the risk of gettin the image side of a photo attached to the surface in touch rises.
With not-so-old photos the treatment is to rise the humidity in order to soften the binder again, with the hope it will come off without harm.
It may be do by a conservator with experience in this things, because even when it goes OK, we’re submitting the picture to a lot of stress. And is always a risky measure.