Ever wonder how do they mix two pictures to show the before and after? it is very simple. All you need is a picture that you have done something on and its original. Here is my first picture.

And here is my processed picture. Using the Move Tool drag it into the first document to have them all in one single file.

Once you drop it there, your document should have two layers.

Using the Rectangular Marquee Tool select a part of the picture that you want to remove to let the original be seen. Look at the picture to get a better idea. Now, click on Edit-Cut.

And here is your before and after picture.

You can cut any place you want.

Or shape.

to get the results you want.