There's tutorials on Youtube that might help you, but I'll tell you anyway.
Go in-game and then to tools>map maker and then to the second stage, and see those two bars up there? You know... defaultTerrain and defaultMask? Type in the full names of the files you want to use for your map. Example: GallTerrain.png and GallMask.png
I know I probably didn't explain it right, but oh well I guess... at least I might have helped a bit? xD