Got it, kinda

This is, however, horribly cludgy. Anyone know of a program, or a plugin
for GIMP, or SOMETHING that will dither a 32 bit image to 16bits for you
and let you SAVE that dithered image?

Hi, I was looking around the GIMP site and apparently they won’t support
depth conversion until version 2.0, unless I am reading something wrong.
However if you have the program xv, you can open your image in xv and
convert it to 8bit by right clicking on the image, then clicking on the
24/8bit button. Don’t ask me why you have to convert it to 8bit for this
to work, but it doesn’t work in 24bit.

From there go click the display button and set it to smooth. You can
then save the image (as a gif, trying to save as png crashed on me, you
might have better luck) and SDL and other programs will display it
without the banding.
Of course you’ll still have to use gimp to convert it to PNG if PNG
crashes on you like it did for me, but upgrading xv will probably solve
that problem for me.
Either way I think this method is a bit more convenient then having to
crop a screenshot. (at least when you have a border around your image.)