Make your bitmap range from pure black to pure white. Black will be sea level, white will be whatever value you plug in when you import (it will ask).
Aren't 30k mountains a little... ummm... excessive? hehe, whatever... it's your map, have fun.
Incidentally, you can re-import that elevation bitmap at any time, in case you want to change it later. Just check for field flatness and water-not-0 after.