[ASM & Hex] Pokeruby: How to edit region map

I don't know how to make a custom image of the region map and then how to import it into the game.

I have found a file possibly related to this:
graphics/pokenav/region_map.png

However the image is scrambled and looks like tiles to construct the map.

How would I go about making my own custom map?

that's not how the region map works. You edit the map directly with a map editor.