The actual worldmap picture is tiled and taken from your master.dat last time I checked. If you want to include your own map, put it in, for example, client/data/art/interface . There you have the default.ini , press ctrl+f and search for the term "worldmap", eventually you'll find it. Now you just rename it to the file you put there before , like "thisiscoolnewworldmap.png" and thats it. Note, that if the map is very large, you need to split it into smaller parts. The relief map you mentionend only determines the, duh, relief and water/mountains etc (unpassable terrain).