Good news!!
As of today,
any JPEG you upload
through the Twitter web interface
will be posted as-is!!
This is good news because it means no more fussing around with special PNGs—just
export your art at max JPEG quality and you're good to go!!
The only catch is, ironically, from now on be sure NOT to upload PNGs to twitter any more,
because it WILL still (horribly) auto-compress PNGs! If you want them to leave your art alone,
you have to provide it as a (high quality) jpeg.
In the future, I plan to add a "convert PNG to max-quality JPEG" tool here,
but in the meantime, most art programs should let you do it pretty easily yourself.
(Also, it looks like they're still making an exception for art under 900px or 256-color palettes,
so despite my warning, pixel-art/binary-pen artists can actually continue using PNG!)
Here's twitter's announcement: