Re: how to add a picture to a layer with a mask ?

You don't need at a mask though a mask can be useful.  If you use menu file>Place.  You can place an image into the current document and use the place transform dialog to size and position the place in image.  The will create a new layer on top of the current image.  You can add a layer mask to the new image layer to virtually crop it, shape it and blend it in.