I've seen this effect before, but what rekindled my interest in it was seeing it on Apple's new iLife product box. If you had to draw the puzzle pieces with the Pen tool, it could be quite a task, but luckily, four puzzle shapes are built right into Photoshop (as Custom Shapes). The technique can be used to make an editorial statement, a dramatic effect, a fun effect—it's pretty much up to you how you use it.
Quick Tip: Erasing back to your original image
If you're working on an image and things start to look bad, you have a few choices. You can go under the File menu and choose Revert, which reverts your image to how it looked when you opened it. But what if you like some of the things you've done thus far ...