[Hummingbird] Optimizing images not very effective…

We ran a performance test on several of our websites and noticed we had a very low score with the image optimization of certain websites: bettybogaers.com, coverjunkie.nl, come2life.nl and freshpeople.nl when we tried to improve our score it seemed to have no effect. The images were already supersmushed and we tried resizing them but all scores remain the same. Could you help us?