How to make your own pathing using GIMP?

[jonbon29]

Hello guys I just downloaded GIMP. I wanna know how to make my own pathing or edit the existing pathing in the MPQ. See picture below.



As you can see guys I wanted to edit the naga stairs, making it longer to suit my needs. How can I achieve that?

 

[WaterKnight]

http://world-editor-tutorials.thehelper.net/pathmaps.php

Modify the image's dimensions?