I want you to introduce this app which makes superimposing really easy. I happened to find this one in Apps store.
It's really easy. Click 'Add Images' and you just choose a screen shot you want to use, or you can drag & drop a screen shot. Then select the model of iPhone and click 'Export'.
Available models are iPhone4, iPhone3GS,iPad2,iPad,iPod touch, but all comes in black.
Vertical shots are superimposed into portrait mode, and horizontal shots are put into landscape mode. This video might be useful to get an idea of how it works.
The price of this app is 5 USD, which is not very cheap, but if you want to display any kinds of screen shots of iOS devices, this definitely adds a new choice for your creation.
How about giving it a try? (I don't mean the girl lol)