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Oh, I thought you were talking about map pieces. Well that's easy. You'd just make a 300x300 image and draw it lined up to your map. The SOM grid is 100x100, so each map piece would be equal to every 3x3 pixels on the image. You can use the map files in the sample game as a reference to help you. Remember to save the image as a 256 color bitmap in your game's pictures folder.
I use Photoshop for all my graphics, but I think a map like that is doable in Paint. Just copy the background color and fill it in, then copy the darkish brown color and start drawing your map lines pixel-art style. If you use Photoshop or Gimp, you can easily give the map lines a faded look by combining stuff like add-noise and a transparent eraser.