optimization - How do I reduce noise in an image?

I have a map image, similar to this:

(click for larger)

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.)

Answer

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.