Sorry to reply to myself but I've got an answer which works.  The code
below puts the raw image into a Frame, scales the original image in it
and captures the output.  It appears that the Image produced is actually
a BufferedImage and so can be written out.

I'd still like to know if there's an easier way that this (which seems a
trifle contrived).

private static Image createWritableImage(Image img)
  {
    int width = img.getWidth(null);
    int height = img.getHeight(null);
    if (width == -1 || height == -1)
    {
      return null;
    }

    Frame f = new Frame();
    f.addNotify();

    // Returning offscreen produces an unclear background.
    Image offscreen = f.createImage( width,height);
    Graphics g = offscreen.getGraphics();

g.drawImage(img.getScaledInstance(width,height,Image.SCALE_DEFAULT),0,0,
null);

    // Clean up

    g.dispose();

    f.removeNotify();
    return offscreen;
}

On Thu, 09 Feb 2006 13:08:56 +0000, Simon Andrews
<simon.andrews@bbsrc.ac.uk> wrote, quoted or indirectly quoted someone
who said :

try System.out.println( image.getClass() );

If you are lucky, you have a BufferedImage just a cast away.