Wikimedia
Wikipedia: Raster Graphics (Bitmap)
Encyclopedia entry that defines what a Raster graphic (Bitmap) is and explains how it works. Includes a nice illustration and an example of how a bitmap picture is constructed. Links to related information are available.
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.