title: Image size optimizing for newer retina screens
body: <p>I need to implement some photo uploading function, but I'm a little bit confused because of the pixel/size dimension of the images. I need to display a square cropped image which size in IB is 320 x 320.  </p>
  
  <p>I've made a little research, but couldn't find an exact answer. I've checked Instagram, and it uses 640 x 640 pixel images everywhere. How is that possible? The iPhone 6 Plus's screen resolution is bigger then the Full Hd, the width of the screen is 1136 px while an Instagram image is 640 px wide (and doesn't pixelated) . I don't think that they save images for every devices separately. </p>
  
  <p>So <code>UIImageView</code> can display non-retina images on retina screens like this? Or what's the reason why the 640 px is enough?</p>