[Bowlingballhead]

Furthermore, I cannot even imagine a way to automate this, unless there is some command in GIMP or Paintshop, "Make the picture background solid black, but don't touch pixels that are obviously part of the unit." Is there a command like this?

Yes and no. Photoshop will perform a 'fill' command with tolerance - ie, it won't just fill in white pixels, but pixels that are *nearly* white, defined as close or as far from perfect color match as you desire. It will also give you fat pencil tips that can be easily used to brush out artifacts around edges.

You may be able to get PhotoDeluxe for free. PhotoDeluxe... is a deliberately hobbled and painful-to-use Version of Photoshop, but it's probably better than MSPaint.