Unfortunately, the newest iOS simulator doesn't play nice yet with AIR 31. So I didn't find a way yet to try out the possible startup image name candidates. :-/
There's no documentation on this from Apple, unfortunately, since one typically uses Xcode's storyboards instead for native development. (Which would be an option for AIR, too, but unfortunately it didn't create an 100% smooth transition in my tests. I only got that working with classic launch images. Besides, not everyone has Xcode at their disposal.)