I've come across this code snippet which extracts the images used by the iPhone OS. You just need to copy paste the code somewhere, e.g. in your delegate, then create the method(selector) signatures in the header file and just execute it when the app finishes loading by calling [self extractArtwork:self];
It takes a couple of seconds to finish, and I would run it from the simulator. You will find all the PNGs in /Users/[you username]/Library/Application Support/iPhone Simulator/User/Applications/[app guid]/Documents/
You can then go through them and see which iPhone OS images you want to use for your app. I suggest you remove the code after doing it once.