How to manage images (dimension-wise) in iPhone OS 3.1x / 3.2 / 4.0?

iPhone is 480 x 320, iPad is 1024 x 768, iOS 4 is 960 x 640. What is the good practice of managing images for 3 resolutions?

Say the app wallpaper as an example, the iPhone is probably a 50%-reduction of the iOS4 one. But it is also tempting to design one dimension (1024 x 768) for both devices (iPad and i开发者_如何学运维OS 4 phones).

Any suggestions? thanks

I would add the biggest size to the app and generate the appropriate image depending on the screen and resolution at first launch. Only the customized images will then be used.

Using a mix of crop and resize should be enough.