Optimizing images by hand is time consuming and painful. There are many good image editing tools that will allow you to get the best visual result for a certain file size but a lot more optimization can be done. Smush.it is a service that uses image format specific non-lossy image optimization tools to squeeze the last bytes out of your images – without changing their look or visual quality.
Smush it comes in different flavors: You can upload pictures in your browser, provide a list of image urls or you can get a Firefox Extension(YSlow) or a cross-browser bookmarklet to optimize the images found on any web page. Smush it also offers an API that performs these optimizations automatically. WordPress users can use WP Smush.it pulgin to automatically run every image added to a page or post through Smush.it behind the scenes.