iPhone 3GS and previous all use 320px(w) x 480px(h) at 160dpi where as the current iPhone 4 'retina' display uses the a higher resolution screen at a pixel density of two. Meaning that there are two pixels used for every one pixel used in previous models giving the iPhone 4 a resolution of 640px(w) x 960px(h) at 320dpi.
Understanding resolution is important when deciding on wall papers as lower resolution images lead to poor quality on the screen. So when selecting a wallpaper you should always ensure that the resolution of the image is always higher or matching to the device that you are applying them to.
Currently there are many free ways to get wallpapers to match almost everyone unique style if you have an iPhone there are many free apps available in the app store and even more wallpapers available free on the web! My personal favorites for iPhone wallpapers are
