Each image that you pull down to the storyboard occupies its own layer. Layering lets you place items on top of each other to compose a complex image or scene out of simpler pieces. You can also use layering to show depth in a storyboard.
Whenever you click on an object, you can change its order with a click of a button. Objects in the background should be behind the objects in the foreground. (PowerPoint supports the same concept.)
Bring to Front
Makes image the top-most image
Brings image forward by one layer
Send to Back
Sends image all the way to the back layer
Sends image back one layer