The SketchClub app is one of the most powerful that I have yet discovered for the iPad. At $1.99 it is extraordinarily inexpensive for the tools in has. It has some great layer tools and the light here was created with a brush very similar to those in OmniSketch.
I created this chair using a few layers to first one to sketch the chair, then paint it and add the other elements. When I can use layers I like to do so because it gives you better tools for erasing unwanted elements without making holes in the image and also for doing things like adding the carpet after you have already done the chair.