Help with adding materials to the toolgun

[Bolagi]

Hi there!

I recently decided to mess about and see what things I could add onto the material tool, I navigated to material.lua as I know that you have to enter a line of code like:
list.Add( "OverrideMaterials", "materialname" ).

I then navigated to materials/spawnicons/models to find the images I'd be using as a material (all in PNG format). I then went to the material.lua that I had open in notepad and entered lines of code like this:
list.Add( "OverrideMaterials", "models/sgg/starwars/clone_commando" )

Now when I'm in singleplayer I see this:

https://files.facepunch.com/forum/upload/285175/4aa821c4-8d7b-4017-b0cc-6279c2a66894/20180627010534_1.jpg

If I select the material and try and place it onto a prop, the prop remains the same.

Any ideas on what I can do? Do I need to replace another material that's already in the game with this one?