Wikimedia
Wikipedia: Png
Encyclopedia entry that defines a PNG graphic file and explains how and why it was created. Sections of the article include history and development, technical details, support and usage, alternatives, see also (related information), and...
Other
Elated Communications: Understanding Image Formats
This tutorial looks at the three common image formats used on the web: GIF, JPEG and PNG. You'll learn how these image formats work, and where to use each type of image in your web pages.