What happens when you paint part of the layer mask
…black? …white? …grey?

When I paint part of the layer mask black it reveals the picture hidden in the back.

When I paint part of the layer mask white it reveals the picture in the picture that was in the front before and got painted black.

When I paint part of the layer mask grey it becomes partially transparent. It makes the front picture transparent so both pictures are visable in a blending way.