This looks like a good resource for you. I'll admit I knew it was in there and it took me a while to find it.
Platform-specific resources
Titanium gives you various ways to include platform-specific resources, like images, stylesheets, and scripts, in your project. Titanium uses an "overrides" system to make it easy to use platform-specific resources. Any file in the platform-specific Resources directories (Resources/android, Resources/iphone, or Resources/mobileweb) will override, or be used in place of, those in the Resources directory. You don't have to use any special notation in your code to specify that these files should be used.
This is just a snippet of the information, there is more in the docs.