Originally Posted by Honf
-bit_depth 4 -plte_len 16 is the exact setting I used. And it produced horrible images (they opened fine in windows, horrible on kindle). I'll try it out again and I'll open up the commands that pngcrush receives in case anyone will feel adventurous and would like to change them or optimize them.
I'd gladly take better default settings though, if you would find them.
I've just released pngcrush-1.7.15 (source only right now) which will automatically reduce the bit depth to 1, 2, or 4 when the plte_len option is used without the bit_depth option, to avoid generating invalid palette entries (which unfortunately won't help you because you did not omit the -bit_depth option). I don't know why the Kindle images would look different from the Windows ones, unless the Kindle is mis-handling sub-8-bit PNG images. If that is the case you might be able to deal with that by using the current version of ImageMagick (IM-6.6.8-6) to write the images as PNG8 and then pngcrush them with no options.