Addy Osmani's ebook makes the case the image optimization is too important to be left to manual processes. All images need optimization and it's the perfect job for automation.
I agree, of course. At the moment I've got a WordPress plugin + Cloudinary one-two punch helping out around here. Optimized images, served with a responsive images syntax, from a CDN that also handles sending the best format according to the browser, is quite a performance improvement.