[Globu]

Beautiful -- worked like a charm.

In case it helps anyone else, after I copied and pasted the white pixels into a new layer (promoting it to new layer), I then took all the white pixels on the original image and used gaussian blur to make them disappear (as it was keeping little diluted white pixels that were right next to the ones I pasted in after reduction).

And it appears that now I can use MapForge! Yes! (It didn't like the 8000x6000 image.) Whew -- what a relief. I was dreading doing all that work from zero in the game's native map editor.

For now, I'm going to work with the assumption that 4000x3000 is a workable size. Is that a good assumption?