Image Optimization Part 1: The Importance of Images
http://yuiblog.com/blog/2008/10/29/imageopt-1/

Stefanov, Stoyan
Yahoo
2008

Abstract:

On average, 46.6% of the page weight for these popular sites consists of images, included either inline with tags or via CSS stylesheets. This is a massive percentage and it tells us one thing: There’s huge potential to improve the performance of websites if we can improve the way we handle the image payload. By focusing on images you can make a difference and delight your site visitors with a faster and more pleasant experience.