"Smush Image Compression and Optimization," which used to be called more commonly "WP SmushIt" by WPMU, optimizes and compresses your images significantly with no noticeable loss in quality. The biggest load most websites carry is media, and when you upload an image to WordPress, it makes even more copies of it, reducing page load speed and loading down your server. This plugin is the best I've found for dealing with that, and there's a Pro version that has some awesome features as well.