Use PNG 1-bit palette

[mmitchell-w]

Have you considered saving PNG using a 1-bit palette depth? In my tests with GIMP, that can save around 66% of the file size versus RGB (256x256).

See http://www.libpng.org/pub/png/book/chapter08.html#png.ch08.div.5.1

We're producing tens of thousands of these at once, so the size reduction would be noticeable to us.