An acronym for Portable Network Graphics, PNG is a graphical file format designed to be patent free. Find more info at http://www.libpng.org/ .
It features lossless compression (see zlib).
The Img package supports this format.
See also: Writing PNG files, Writing PNG Comments, Reading PNG image dimensions, Reading PNG Comments, Reading PNG Timestamps.
As of 22 May 2004, the png package is a tcllib module that incorporates the above functions plus a few more. Documentation can be found at http://tcllib.sourceforge.net/doc/png.html