GIF
News and Media, Burn All GIFs, GIF Economy: A Cautionary Tale, Graphics Interchange Format, Graphics Interchange Format, The gif2png Home Page, Unisys/CompuServe GIF Controversy, US4558302 Patent
JPEG
Extensions of jpegtran, Implementation of the JPEG2000 Standard in Java, Independent JPEG Group, JPEG FAQ, Online JPEG Compressor, Small JPEG Decoder Library
JPEG2000
Esprit Working Group 27100, JasPer Project (JPEG-2000 Software), JPEG Vs. JPEG2000 Comparison by Amey Purandare, JPEG2000, LEADTOOLS JPEG2000 Plug-in for Developers, Migrator 2000, Morgan JPEG Toolbox
PNG
Explorer versus PNG (AlphaImageLoader), MSDN - AlphaImageLoader Filter, Open Source Graphics with PNG, PNG, PNG Resources of the Miscellaneous Persuasion, PNG Specification, PNG Tips for Cartoonists, PNG-Tech, Portable Network Graphics - PNG, The Sad Story of PNG Gamma Correction
Graphics Interchange Format Specifications
Specifications to various graphics formats, including GIF, JPG, DWG, PCX and several others, from Edinburgh University.
JPG vs GIF
A comprehensive article on the differences between the file formats.