This is a really nice idea. In essence:
Pictorical are a publishing house, who sell ebooks for iPad. You apply to register with them as an artist and –provided they like your illustration style– they sign you up and you choose from their list of available books and illustrate it. When it’s done, they sell it as an eBook and you [as the artist] get half the profits.
Applications are closed at the moment, but it might be one to keep an eye on for the future.