Smushit is another plugin that allows you to address you blog load time by directly addressing image size.
Once it is installed it works automatically in the background and basically its aim is to remove unnecessary data from images and reduce them to the smallest file size possible while still retaining the quality. Recommend by Yahoo itself in the Yahoo exceptional performance series this plugin is a must have.
It works by stripping out small, if not minute, pieces of code in the images that have no real need to be there or which are unused and also may suggest converting the images to gifs, if required, although caution should be taken here incase quality is unintentionally affected.
Download the Smushit Plugin for WordPress.