Layering Images.

The two images are both in different windows of photoshop, both on different layers. 

To get them both in the same window but on separate layers you click on and drag the image that you want to be on the top over to the other image and drop it so as it is now in the window.

The top image has now been sized to match the bottom image and has been hidden from view as it is the background layer. 

Both of the images are on separate layers and can now be changed independently.