app.js

// Assuming you have the images following images, this should work fine
// images/background@2x.png
// images/background-568h@2x.png

require("ImageHelper");

var window = Titanium.UI.createWindow({
		title: "My awesome window",
		backgroundImage: parse568hImages("/images/background.png")
});

ImageHelper.js

// I am pretty sure there is a sexier approach, but this works nicely
var isPhone5 = Ti.Platform.displayCaps.platformHeight == 568;

parse568hImages = function(imagePath) {
	if (isPhone5){
		// Do this because the filename could contain dots
		imagePath = imagePath.replace(".png","-568h@2x.png");
		imagePath = imagePath.replace(".jpg","-568h@2x.jpg");
		imagePath = imagePath.replace(".jpeg","-568h@2x.jpeg");
	}
	
	return imagePath;
};