title: About IPhone4 screen resolution
body: <p>I read from <a href="http://www.apple.com/iphone/specs.html" rel="noreferrer">http://www.apple.com/iphone/specs.html</a> that IPhone4's screen is 960-by-640-pixel resolution at 326 ppi.</p>
  
  <p>But in Xcode's IPhone4.3 simulator, when I manipulate the display objects, print the [UIScreen mainScreen].applicationFrame, it is 320*480.</p>
  
  <p>So if I wanna use a picture as the main screen's background of my app, which size I should use? 640*960 or 320*480? </p>
  
  <p>Or Which kind of images should I use ? The size of images can affect the details a lot.</p>