The Graphics File Format Page - Specifications for various graphics file formats along with some source code and utilities.
Graphics for Inclusion in Electronic Documents - Essay on how to produce portable graphics files that can be imported into other documents, especially TeX. The advice is aimed mostly at linux or similar operating systems which have open-source command-line tools. Also includes guidelines for image producers.
Image File Formats - Listing of image formats and their associated file extensions.
Image Formats - Information about seven common web image formats.
The Unofficial Photoshop Color Book File Format - Adobe Photoshop stores color books in a native format with the file extension .acb on PCs and 8BCB file types on Macs. This is an unofficial documentation of the specification.
W3C Overview of Web Graphics - Information from the World Wide Web Consortium on format comparisons, integration into HTML and XML.