Two more:
Same as what's posted on vgmaps but ~100 KB smaller.
396 KB quarter resolution 256 color dithered version of the above, this time downscaled with fant rather than bicubic. At extreme zoom, fant handles the small boxes more gracefully than bicubic. It also makes for a slightly smaller file size.
I used Paint.NET for downsizing, but GIMP should be competitive. GIMP also has good support for color indexing.
As for dithering, it's used to fake increased color bandwidth through a method called error diffusion. The algorithm effectively injects digital noise into the image, and the end result is it tricks our eyes into thinking there are more colors than actually present. However, being that random data is being inserted, dithering harms compression and is rarely useful under 256 colors compared to increasing actual color bandwidth, assuming that the original image uses 256 or more colors.
TruePNG (and probably most other quantizers) sacrifices the purple region because it is one of the least prevalent colors. Other regions of the map also drop out as the color bandwidth decreases. Keep in mind that any scaling method other than nearest neighbor increases color bandwidth via smoothing.
Here's a 24 color dithered version of the full size image. As you can see, there isn't enough color bandwidth to represent objects in the southwest purple region and objects have been replaced by a speckle pattern, otherwise known as dithering. Despite the lack of color bandwidth, dithering drives file size to a hefty 788 KB, the same size as 256 color non-dithered, which is far more appealing looking.
