Graphics on the Web
http://www.w3.org/Graphics/

W3C

Abstract:

There is no limit in the Web specifications to the graphical formats that can be used on the Web. You just need a MIME type so that the format is labelled correctly for transfer across the Web, and so that a suitable viewer (if one exists) can be located at the other end. In practice, certain formats are more widely understood than others; certain formats are more suited to one type of graphical data than another; so you should make an informed choice about what format to use.