Before and after numbers from meowapps.com: routing every image URL through Easy IO with Perfect Images cut the images our server sends by 73%, and what reaches visitors from the server by 86%. Where the leaks are, and how to do it.
How to: Optimize Images in WordPress
How to optimize images or which CDN to use with WordPress? The simplest way is always the best.
Learn: The WordPress Images Sizes & The Upload Process
How are the thumbnails created? What are those WordPress Images Sizes?