I'm building an app with Appcelerator Titanium that will run on iPhone, iPad and Android. With Titanium it's quite easy to distribute iPhone- and Android-specific images - custom map pins, native-looking icons, that sort of thing - but I've only now just figured out how to distribute iPad-specific images aswell. So for anyone else who's struggling, here's how you do it. (I found the answer through a ticket raised on the Titanium issue tracker). Rename your iPad images so that the text ~ipad appears just before the file extension (so mapmarker.png becomes mapmarker~ipad.png, and firstname.lastname@example.org becomes mapmarker@2x~ipad.png), then place them in the Resources/iphone directory. For clarity, it's a tilde (~) symbol, hot a hyphen (-).
This is inline with the iOS guidelines, but slightly at odds with the (incorrect but understandable) assumption that you'd put iPad-specific images in a directory called Resources/ipad.