[hinkley]

The PNG spec is an interesting read. There are some overly fancy things in there and some dead-practical ones, like the magic number at the front of the file contains an 8 bit character ("8 bit clean" was still a phrase you could utter back then, today it's assumed), the section headers are plain text (you can run 'strings' on it), and there's even a bit that determines whether the section is mandatory to render the file properly. You can add arbitrary metadata and any other reader can still display the image.