How is it making use of tiles? I don't really see any repeating elements.

"Tilemap" palette editing mode
Analogous to on-canvas tile editing in tilemap-aware pixel editors (tilemap-and-tilesets being abstractly identical to indexed-images-and-palettes).
Painting on canvas doesn't change pixel indexes but changes the palette values instead.
Would need either a second palette or RGB[A] painting for source colours.
Could be useful for colour theming perhaps?
Or useless?

The green of the foreground grass is rather too intense for the rest of the scene, could do to be toned down some.

Is the grass surface on the background earth walkable? If so I'd try to make it closer to the foreground grass in style and colour.

The distant background layers could be made more interesting by having parts of the further layers dip behind the nearer layers so they don't all form continuous bands.

Now you've got some screen shots:
Pure magenta is just about the worst possible choice for a background colour. Hard on the eyes and colour polluting (colour perception being relative). There's good reason that low-contrast grey checks are the de facto default. I do hope it is at least configurable to something else.

Given that you are selling it, what does it do that's better, or at least different, from existing tools? The listed features are seem pretty standard (though not sure what "flood shade" means). Why, for example, would I choose to buy this over using GraphicsGale for free?

The max image size recommendation seems arbitrarily limiting and doesn't inspire confidence.

Wine doesn't seem to like it, so I haven't been able to try it (couldn't be arsed booting into windows), so I can't comment further.