You know what a map in a videogame is? Not just a drawing like the Kalimdor map you posted but a plane or space, with a coordinate system, in which objects and characters can move. In WoW there are different maps, EK, Kalimdor and Outland as the large ones and smaller maps like instances, battlegrounds... You get a loading screen when you change to a different map, for that you have instance portals or other triggers. That's why no one, not even troll priests, can swim from Teldrassil to Azuremyst, becoause the latter is on the Outland map.