If you often find yourself bringing various images together in Photoshop (and I predict you will), you can't simply toss the images together and have the new image look realistic. As you've seen, effects such as shadows are essential to create a realistic-looking environment. Creating reflections is another technique for doing this.
Try it Yourself
Add Reflections to a Surface for Realism
Let's look at an example to see how you can add reflections to your toolkit of special effects: