Import an 256x256 blp file with any MPQ editor and name it war3mapMap. Also remove the war3map.mmp file. Be sure to do this after you are done editing the map because saving the map will overwrite the war3mapMap file.
Now, if you want to keep the original minimap in game, first save the original war3mapMap file somewhere before overwriting it with your custom one and then reimport the file into your map under a different path. Then at map initialization, do call BlzChangeMinimapTerrainTex("original\\minimap\\path").