UIImage: make an image look like a shadow

Question

How do I make a UIImageView show the "shadow" of the the UIImage in it?

I have a dog.png file, but I just want to show the shadow of the UIImage so when I press a button it reveals the real image of the dog. so I want something like this:

and after I press the button It shows the real Image:

Please give me a hand on this.