What is it
PNG is an extensible file format for the lossless, portable, well-compressed storage of raster images. PNG provides a patent-free replacement for GIF and can also replace many common uses of TIFF. Indexed-color, grayscale, and truecolor images are supported, plus an optional alpha channel for transparency. Sample depths range from 1 to 16 bits per component (up to 48bit images for RGB, or 64bit for RGBA).
Status
The PNG specification was first issued as a W3C Recommendation on 1st October, 1996 (press release) and updated to a second edition incorporating all errata on 10 November 2003. This edition is also an ISO standard, ISO/IEC 15948:2003 (E).
This means it is a mature document that is considered to contribute towards realising the full potential of the Web. Viewers for PNG are available on many platforms; there are an increasing number of content creation tools available; and thus modern browsers implement support for it also.
MIME type
The MIME type for PNG, approved on 14 October 1996, is
image/png
More information
The official PNG home page is maintained by Greg Roelofs and includes a frequently updated listing of Web browsers that support PNG (including plug-ins), helper applications, and content creation tools (both interactive editors and conversion tools).
You can also read The Story of PNG
If you are not sure just what the gamma and chromaticity features actually mean in real life, look at this description to see how they contribute towards improved cross-platform graphics. PNG also has a novel interlacing scheme which provides a usable graphic faster; and the inclusion of metadata in the file means that search engines can find graphics based on their descriptions rather than their filenames.
Browser testing
See if your browser supports PNG and if so, whether it handles gamma and alpha correctly.
