title: Do PNGs (or JPGs) have a DPI? Or is it irrelevant when building for retina?
body: <p>A simple question that I have been having great difficulty finding a definitive answer to: do PNG files have a DPI?  Or perhaps more importantly, is it even relevant when building retina-enabled sites/apps?</p>
  
  <p>I've just received PSD assets from our designer for a retina iPad app that I must convert into HTML for display within the app.  Typically, I receive such files as 2048x1536 @ 72 DPI -- double size but standard screen DPI.  I then typically use CSS to tell the browser how to display it.</p>
  
  <p>But this time the designer was instructed to provide his PSDs at 1024x768 @ 144 DPI (standard size but double DPI.)  I believe this is incorrect, as the DPI setting within Photoshop is intended for print purposes.  Plus, when I save something from a 144 DPI PSD as a PNG or JPG, it is exactly the same as one saved from a 72 DPI (or 30,000 DPI for that matter) PSD.  DPI doesn't seem to be reflected in either any setting that I can see in the resultant file nor in a different file size. It seems, at best, metadata.</p>
  
  <p>So, it's my understanding that DPI isn't relevant here, and we should simply be asking for double-sized assets for retina projects, but I would like some confirmation/clarity on the issue before asking for new assets.  I work with many designers that are transitioning from print backgrounds so this is a common issue I encounter and I'd like to be able to provide better guidance with our requirements in the future.</p>