In my app there are folders drawable-mdpi (for phones) and drawable-sw600dp-mdpi (for tablets). When i launch application tablet takes images from folder drawable-mdpi. But i think it's wrong behavior.
My device is Acer-A500 (1280x800 mdpi). How does android choose png?
Acer-A500 was launched with Android 3.0 the identifier sw600dp didn't exist back then, so the tablet doesn't know it exists.
Newer devices shouldn't have problem finding it. You can check by using an emulator.