It doesn't seem like it's possible to display an image in its place. Not according to the information here anyway.
Check out the "Hand Edit the Navigation Map" section.
My current WIP map uses the height limit increase that's possible with the modded editor. It causes objects to display wrong on the in-game map though and that's why I'm trying to change this image. Or at least trying to figure out how to get the game to display the map properly. If you discover anything, please share it here as it may help to find a solution to my issue also. Thanks.