Most images you see on the Web are either GIFs or JPEGs. Each format has its own advantages and disadvantages, and learning when to use each one can make your Web pages look better and download faster. Another format, Portable Network Graphics (PNG), promises to bring even more flexibility and functionality to the Web, and is likely to be adopted in the next round of browser updates. Here's where to get the latest on image file formats.

General Background
Graphics formats for the World Wide Web
As with most Web technologies, the World-Wide Web Consortium (W3C) is a great place to start.
File formats FAQ
Links to graphics file format frequently asked questions from Usenet.
GIF or JPEG?
Adobe Systems gives advice on how to make the smallest, best-looking images for the Web, with lots of comparative images.
Preparing Graphics for the Web
Useful notes on file formats from Douglas E. Gray.

GIF (Graphics Interchange Format )
GIF-87 specifications
CompuServe's original 1987 specification.
GIF-89a specs
The updated (1989) specification, which also covers animated GIFs.

JPEG JFIF (JPEG File Interchange Format)
JPEG File Interchange Format Specification
The W3C's official specifiaction.
The JPEG FAQ
Frequently asked questions about JPEG compression, from the Usenet newsgroups.
The JPEG Playground
Programming resources, links to software and specifications, and other JPEG-related information.

PNG (Portable Network Graphics)
Overview of PNG
From the W3C, an overview of the format, along with the specification itself.
PNG Home Pages
Useful information and links about the PNG specification, maintained by Greg Roelofs.
PNG Documentation
Complete documentation in a variety of formats.
Graphics Formats: GIF and PNG
A comparison of the GIF and PNG formats.

Other image formats
There are dozens of unique image formats used only infrequently on the Web. Here are pointers to additional resources for formats not covered above..
CICA' Image File Formats List
Although not updated in a while, this lists most common image file formats created by popular graphics software.
Yahoo!: Internet Data Formats
A frequently updated list of related Web sites.