also known as the tile circled here:

with this one: 
The replacement is done in your tiles.bmp file inside your arc directory, probably:
c:\windows\program files\sierra\arc\tiles.bmp
It is tile number 817 in your map editor and the tile starts at pixel 272, 320. You should be able to copy and paste it right from the picture above, or you can download it or whatever.
Also, if you are lazy, you can download the tiles.bmp file with the tile replacement right here.
Here is a screenshot without the tile:


The overworld picture of Gencrux on the story page was created with ARCMap which you can find here. ARCMap is copyright of RagnarSoft and the creator is a really, totally awesome person who hopefully won't sue me for using ARCMap to create the map!
The current version of the map is available here.