Smushit.com is a website which claims to do the same task. It is an online Image processing engine that optimizes the images, and saves you every bit of data, without any loss in the quality of the Image. We tried compressing the header of the Blog, which had been optimized for web by Photoshop, and still it saved 27 % of 4 KB header logo. Good enough.
Here are the logos before and after compression.
Not quite a loss in Image quality, and a big save of 27%.
The best part is that it also comes as a Firefox extension, and you can optimize the web images straight away from the extension itself. So go trying for the Image optimization, and save those precious bits and bytes going waste with your images.