Attached Link: http://www.libpng.org/pub/png/pngintro.html

"Since PNG's compression is fully lossless--and since it supports up to 48-bit truecolor or 16-bit grayscale--saving, restoring and re-saving an image will not degrade its quality, unlike standard JPEG (even at its highest quality settings).".... from http://www.libpng.org/pub/png/pngintro.html