(keitai-l) J-Phone grayscale PNG image format

I want to ensure that our images are properly rendered for J-Phones that do
not support color. We could render the images in grayscale in either an
indexed PNG with a grayscale palette -OR- using the specific grayscale
format (PNG color type 0), in bit depths of 1,2,4,8, or 16. We do not know
which format J-Phones prefer or require, but it appears that the indexed PNG
would yield a smaller file under some conditions.

Thanks in advance,

Brian Hutchison

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