Personal Camera is an app that lets you take photos and videos using an image of your choice as what you see on the screen. You can even take a screen shot of your home screen and use it.
Select any image you want, including an image of your home screen if you want to psyche your buddies out, and tap on the icon to take a picture (or video if you have an iPhone 3Gs).
This app drops the images or videos into your camera roll so theres no complicated exporting like there is with other apps.
Easy as that.