At left, you'll see the image I purchased at istockphoto.com has a dark background. Derek Murphy of Creativeindie Covers used this graphic to produce the cover for The Bad Death. At the time, I didn't have a large file of Derek's work. I used a jpeg of the cover in progress, pictured right. To change the model's brown eyes to my heroine's gold, I used Photoshop's marquee tool to select the gold eyes from Derek's small file and copied them into my large file, resizing them to fit my stock image.
With the path selected, I clicked on Image in Photoshop's top menu. From the Image dropdown menu, I chose "Apply Image". At that point a palette popped up allowing me to select the layer containing the background.