title: The resolution of pictures of IOS app
body: <p>My current resolution of all the pictures (all of png format) in my app is 300, which I realized is too high,</p>
  
  <p>I heard that 72 ppi will be enough, but however I just wanna make the best vision effect of these pictures,</p>
  
  <p>so what is the best resolution of pictures for IOS app? both Retina and former screens.</p>
  
  <p>And one more question, I tried to save my pictures in photoShop to 72 ppi,</p>
  
  <p>but after I drag that picture to my XCode project, XCode tells that the resolution is 71 pixels/inch,</p>
  
  <p>would it be a problem?</p>
  
  <p>THanks a lot!</p>