So here's what I've done. I have a lovely picture of Abby on a plain blue wall and a macro photo of a rock I took at the Smithsonia Museum of Natural History.
Duplicate the Abby photo layer (Ctrl J). I brought the rock photo layer into the Abby photo and placed it below the Abby duplicate layer. Then I masked out the background of the Abby photo which revealed the rock photo below.
I did do several other adjustments to the Abby photo including removing a few blemishes, whiting the whites of her eyes, and altering the color of her irises to go with the color of the rock.
I played with this a little more with a great photo I had of Levi and a macro photo of Citrine. This time I changed the color of the rock photo (using a Hue/Saturation layer over the rock photo) to go with Levi's eyes.