I’ve been testing kraken.io Image Optimizer and so far color me impressed. I have images that have been reduce in size by 70-80% and there is no noticeable difference between the the original and the optimized image. Amazing. Mind you they are a paid service, but they give you a quota of 100MB for free to test their service, which I am taking full advantage off. The pricing start at $5/month for 500 MB of images per month which is more then plenty for a small site.