Because of the limited color palette, certain images look bad when created using ImageOnMap. Dithering can help a lot with this. Here is comparison between ImageOnMap and this tool with dithering enabled: (image used: click)

Of course, dithering needs to be optional, because many images look better without it.

Cool feature, what dithering do they used ? I know there are a lot of different dithering

Well, you can also look at this tool. It is open source and offers four algorithms. Floyd–Steinberg seems to usually give the best results when an image needs dithering