Yahoo recommends a number of things to improve web page performance. There’s a couple of WordPress plugins that help with image performance.
WP Smush.it
Yahoo has a service called Smush.it that optimizes images. The WordPress plugin, WP Smush.it, reduces the file size of most images added to posts and pages automatically behind the scenes. It’s non-lossy, so the look and quality of images isn’t altered. Through the Media Library, existing images can be run through Smush.it.
Image Pro
Something else Yahoo recommends is not scaling images using HTML code. The plugin Image Pro lets you resize images in the editor, by dragging the corners, like you’d do in Word. The resized version is saved as a separate image so in the post it isn’t scaled using HTML. However, sometimes there’s a slight quality loss using the plugin.