Querying the pixel density of the device is not the same as describing the pixel density of the resource. This has been a common misconception in the discussion of this feature.
What it's representing is not necessarily an intrinsic property of the image but rather the intended pixel density for presentation. Say I have a 600x200 PNG which fits my design for normal resolution displays where 1 CSS pixel = 1 device pixel (which is most current laptop screens, for instance). But then I'd like to take advantage of the extra resolution on a device that displays at 1 CSS pixel = 2 device pixels, such as a retina display iPad. So then I get a 1200x400 version of the same image (perhaps both downscaled from a super high res original) and serve it as the 2x image. It will take the same logical space in the layout but provide extra quality on a higher resolution display. There's no need to do anything special to the PNG.