Picstitch Tutorial?

None that I know of. Google is your friend.

Also check stack overflow, github, SourceForge, etc.

Tutorials tend to be limited to small snippets of code to show you how to complete a single task.

I could pretty easily put together a tutorial that would show how to combine several images onto a larger "canvas" but it would be a low-level tutorial on how to create a graphics context and draw images into it. I would not include all the UI you would need for a collage app like selecting images from the camera roll, letting the user drag around the images before he generated the collage, displaying previews, a UI for saving/printing the output, etc.

Creating a full featured application ends up being pretty involved.