Re: [Gimp-user] extract a reasonably dark image from its background?

[James <virtue rocketmonkeys com>]

I'd recommend doing Color > Levels first, then making sure the black
is as black as you want.  Otherwise if it's dark gray (not quite
black), that will translate to partially-transparent when you do the
Color to Alpha.

You could also do Color > Levels and adjust alpha layer after.  Either way.

-James