PPI can be stored in the "pHYs" chunk in a PNG. And some image processing/editing tools do so, such as Photoshop starting with version 7. http://www.libpng.or

PPI can be stored in the "pHYs" chunk in a PNG. And some image processing/editing tools do so, such as Photoshop starting with version 7. http://www.libpng.org/pub/png/book/chapter11.html#png.ch11.div.8 http://www.libpng.org/pub/png/book/chapter11.html#png.ch11.d... http://www.scantips.com/basics9p.html http://www.scantips.com/basics9p.html