How to make a Wallpaper app for iPhone in XCode?

I'm trying to make a simple Wallpaper app for iPhone in XCode. Wherein i have a bunch of images in my app, user opens an image that they like, and there is a "Set as Background Image" button. I have managed to make a library of images. How do i add the "Set as Background Image" funtionality? Please help. I am grateful for any amount of help that i will get.