Thanks, for mentioning my app, AutoScreenshot 3D!
I created it for myself, as well as other developers, who want to superimpose their screenshot onto an actual image of an iOS device, and have it displayed at an angle, with mirror effect. Makes a nice screenshot for iTunes or web or print!
The PRO version, for $0.99, lets you provide your own background image. By default, it ships with an iPhone 4. You can also download images that you can use for free, from http://www.AutoScreenshot.com. We will add iPad images in the future, but you can google for an acceptable one.
A free version is awaiting Apple's approval and it only uses the default background.