Here we will learn how to mask images or you can say how to combine two pictures. Okay, first of all open two images which you want to mask. I will use the pictures below. You are welcome to use them for this tutorial if you want.
Copy the second picture and paste it on the first picture.
Choose the Rectangular Marquee Tool (M) and change the feather to 30 px. Actually it depends on what size of picture you are using. If you are using larger images you should use more pixel for feather.
Make a selection like below with the Rectangular tool, now
Hit DELETE key of your keyboard and VOILA you masked your images. Here's what I came up with,
You could keep pressing the DELETE key until you see what you expected.