I have a map image, similar to this:
At 100%, the ocean is noisy and therefore not effectively compressed to png.
Please point me in a direction of an intelligent tool for the job. (I tried GIMP but failed.)
You could use Vector Magic, which is a pretty awesome service that converts images to clean vector art.
I tested it with your image and the result is much better than the original JPG version and half of it’s size.