for the game only the texture name matters (as long as it find it's ok)
it might be missing from the Zortch/data/mapskin folder
I made a handy guide (also includes the steps you are familiar with - just a draft for a future editor manual)
(and of course I still need to fix it to make this easier - sorry for taking so long)
Don't worry about it, take your time to work on it ;)
Something strange I've noticed tho is that my Zortch files are not in the same place as you showed them to be. This is strange, since the game running hasn't had any problem with it. This is how my files look on the main folder:
And on the editor folder:
Could this be the reason why the custom textures doesn't seem to work regardless of your guide?
