You have to select the part of the picture you want to cut and paste in to the other picture very carefully. What I do is make the picture very large and use the lasso tool to outline the part I'm copying -- and it's not easy when you are dealing with people sometimes. Once you've gotten the are selected, you can "Control C" to copy.

Have the picture you are copying in to open (and it should be the same size dpi-wize). Click anywhere on the destination picture and "Control V" to paste. Then use the move tool to position the pasted content in the right place in the new layer that is created.

Once it's placed properly, you can merge your layers and you've got a new picture.

Hope this helps.