iPad paintings as preliminaries for paintings on canvas- Seattle, WA

I have been using the iPad application "Brushes" to create preliminary color sketches of paintings coming up. These are two versions of a painting for a book project that I am working on with a writer/photographer. I've started the painting, and so far it's not either of these arrangements. I've yet to see how useful the iPad will be as a tool. It would be perfect if the layers could be moved around and re-sized. Anyone know of an iPad application that allows for more manipulation of individual layers? I can move the image to Photoshop, but would love to do it all in one application.