In the never-ending bouncing back and forth between iPad drawing apps, I think I'm leaning towards investing more time into Concepts.app. Its vector based (meaning images scale up easier) and has an infinite canvas (you're not restricted to a certain size until you export).

Plus, while I don't believe it's open source, it is by a small indie team, called TopHatch. Btw, I wonder if that company name is a reference to LOST.