[ale] Best way to 16-color picture?

[ale at FultonGreen.com (Fulton Green)]

And to answer your first question :), it's for a shortcut icon for my
website, which seems to break in most browsers if my 16x16 icon uses more
than 16 colors.

Right now, I have an extremely shrunken-down version of my driver's license
photo that's using 24-bit color (or whatever color depth ppmtowinicon might
shrink it down to). I tried to get it down to 4-bit color using the GIMP,
but I didn't seem to find a really obvious colormap shrinking function. Can
any of you all give some direction, or should I use something more along
the lines of ImageMagick?

TIA

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