Start with a new document. I user 800px X 800px. Make your background transparent!
Insert the image you want to add your background to!
Grab the gradient tool and make get these settings:
Now apply the gradient like below:
Well that's it. Now just add some shadow and a reflection and you could end up with this:

10 Random Photoshop Special Effects :
10 Random TutorialKit Materials:






More Photoshop: