The map preview is generated by the "draw" scripts -- drawMap.sh for biomes, terrain, water and roads followed by drawPrefabs.sh for prefabs. There's some other stuff there for variants. It's basically ImageMagick commands plus some extra stuff to convert prefabs.xml into an input to ImageMagick, so I thought it might give some ideas.
The main difference between the maps we generate is that I use the prefab preview image to display the prefabs, with a border of tag-based color coding. The logic for color choice is on prefabZoningColor.sh and there's a legend.png with all the colors I use. I do like the icon system of your generator, but I think it could be enhanced with the use of zoning colors like mine.
I'm curious about the algorithm you used to display elevation changes -- that's the one thing I was never completely happy with. I'll check out the code, though pointers would be appreciated. Here's what the sort of preview generated by my tool, for a comparison.