Max, check out the amazing app, iColorama (for the iPad). The iPhone version is iColoramaS, but it really is best with the iPad real estate. The masking (with lots of choices of brush textures/opacity, and the blending functions are mind blowingly exceptional. A time little investment in learning the app will reward you greatly.
Ive had just as much fun as you :) I've been playing with the "123d Catch" app.. Its a free 3d scanner, and you can also edit on web http://www.123dapp.com/catch
Care for some chokolate cake? (scanned yesterday)
I know you can do this in Photoshop -- also in Paint.net -- but Photoshop costs way more than $2.99 and even though Paint.net is free it's pretty complicated (and I don't think it runs on an iPad).
Things like Paint.net are full-blown graphics packages -- Image Blender focuses on one simple function that is easy and intuitive to use.
Clive, this is a standard function in Photoshop and its free cousin GIMP. You'd make a selection based on a shape or color, or select a shape to fill manually, switch to a different layer and paste another image using alpha transparency.