Drop your scheme1.png (1024x1024) in this folder, next to this file, as:

  graphics/schematics/scheme1.png

That's the only path referenced by prototypes/entities.lua. Nothing else needs
to change - the entity is already set up to scale the 1024x1024 source down
to fit a 12x12 tile footprint (384x384px at 32px/tile) when placed.

Delete this .txt file once the real image is in place (Factorio doesn't care
either way, it's just a placeholder note).
