Superimpose is free in the app store right now. Its a good way to combine multiple images together. It’s similar to ImageBlender, but has more selection tools. Of course, more tools and options makes for a more complicated user interface, but it looks like a very good app after using it for about 20 minutes.
This was my first attempt with the app. I’m sure the mask could have been done a little more cleanly, but I haven’t figured out all of the controls yet.