A few weeks ago I've implemented a really interessting customer order, I really looked forward. For the new iOS app the customer needed a "photo theatre" component where the user can choose a so called coulisse. In this app a coulisse is an image e.g. a 80's haircut, animal costume etc. with a transparent area. Then the user can choose either a photo from the image gallery or he can take a photo with the camera. If both images are selected, he can move, scale and rotate the taken photo as long as it fits into the coulisse. If the user is satisfied with the result, he can save the image the app's gallery.
First I did some research because I had no idea how to implement the image manipulation by using the iOS SDK. So I found this really useful article from Roger Chapman that look perfectly for my problem.