Check out this vid for an explanation:
I started with these two images:
You can achieve this result by dragging the pictures over eachother and using a Layer mask. After you've dragged one image over the other, paint with an opposite colour to make things visible or invisible.
When you paint on a maks always rember the following rule
White reveals, black conceals
Have a go! I found this technique to help me in improving my masking skills.