Imported gltf models, created mesh lib for gridmap - how to use single PNG file?

Godot Version

v4.4.stable.official [4c311cbee]

Question

Hi, I have a very basic question:

In GitHub - afarber/godotcraft at dfe17d263bab958c2432ba2a13760566020cb25c I have imported gltf models for cubes and they all come with identical PNG file.

Then I have created a mesh library and use it in a gridmap.

How to change this so that a single PNG file is used for all bricks?

And would it save any memory? Should I use my text editor to change the filenames in the godocraft/scenes/blocks.tscn or is there a better way?

I guess what would be good is maybe to have 1 tres material file, and then I also would be able to increase the roughness of all blocks at once by editing it.

My problem is that when I click around in my Godot project right now, there is nowhere a GUI saying "yes, I use Grass_Blocks_PixelArt.png file and I use a particular cells (like in a sprite sheet) inside that file:

And that means I cannot create a new resource file, add the PNG file to it and then edit all cubes to use it

Thanks

I have tried exporting just a single resources/blocks_material.tres

But now my gras cube has the green gras on its side and not on the top