How do I import an image into a tile map, for the map?

So for complex reasons I'm using a tilemap with 1x1 tiles that correspond to different colored pixels as terrain in a game.

How would I bring in an externally created image to the map part of the tile map? Designing them in Tiled or the built-in map editor is... implausible at any large size. :-/

You could use the canvas object to get the pixel colors.

...I can't believe I missed that.

In reality, what I wanted was available allll the way back here:

Also thanks to you, R0J0hound!