What iPhone app or computer program can you add animal faces to your picture?

Like a tiger or a lion, I' see people do it all the time but I want to know how.
I hate to use this as a description but, it's very indie looking. Like you just ad the face of a lion to the picture to "replace" yours

I don't know if there's an app like that in the app store, but the only thing I can think of is photoshop