They specifically mentioned that they have PNGs of logos that they avoid converting if they're <300kb. Optipng would work well on the images that they keep in PNG format.
They should try just compressing them with mozjpeg anyway, since mozjpeg has deringing https://calendar.perfplanet.com/2014/mozjpeg-3-0/ it's actually not bad for text/cartoons/other things with high-contrast edges
