Before you down vote, bare with me as I’m a developer! I’ve always been curious how designers render their work onto a phone to show their customers how it may look on the phone, for example:
I’m assuming that’s exactly what they’re doing, using some kind of 3D rendering of an iPhone and transforming their image to fit the screen. Any tutorials would be great… just satisfying my curiosity, thanks.
Search around for “mockup template”. There are some free ones out there, and some paid. These allow you to easily create a realistic mockup using whatever artwork you desire.
Here’s one good resource: http://www.pixeden.com/psd-mock-up-templates
Related question: How can I take an iPad psd file and put an image inside it?.