To reiterate what Rick said - a PNG isn't always a PNG. Or to
paraphrase George Orwell, all PNGs are created equal, but some are
more equal than others.
I don't know that the PNG format has been standardized. The
compression algorithm seems to differ depending on source. I can
take the same bitmap and convert it to PNG with different graphic
tools and get very different files (up to 15% variation in size).
Some of these PNGs work in CHM files, some don't. They all seem to
work across graphic viewers as far as I can tell but other programs
- like Word, RoboHelp, and the help viewer (IE components) only
read some of them.
John