The image file is a height map. Darker is lower elevation and lighter (white) is higher elevation. The Map1 folder seems to be the "zoomed out" map image and the Map2 folder holds the "zoomed in" map images and in-game terrain information.
I suppose it depends on what you are trying to accomplish. If you wanted to add a street map or satellite imagery, for example, it won't work. If you want to add/edit terrain features like canals, search the forum as there are tools for this.
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