Look here:
Real Navigation - maps with lighthouses
Or my method. Make one graphics file with, let it be a yellow dot in the center.
Name the file Lighthouse.tga. Place it in terrain\location folder, there have a file Lighthouse.dat.
Now the lighthouses will be visible on map, according to zoom parameter "HarborZoom=" in "data\Menu\cfg\Maps.cfg" and "ShowOnMap=true" in "terrain\Locations.cfg".