Saving an image as “grayscale” doesn’t simply mean it’s supposed to have only versions of “gray” in it’s palette. Actually it is not even allowed to have that palette.
And it is also not about “color depth”. “Grayscale” comes in several possible amounts of colors, 2, 4, 8 ,16, 256.
Cut the crap – how is that funky image converted into “grayscale” ?
The tool of joice is “irfanView” (at least on Windows; XnView supposingly on Mac? Comments welcome).
Now the important part – it is NOT about “Menu > Image > Convert to Grayscale”. This is only half of the business (or not even necessarily needed).
The magic comes when you press “Save as .. > PNG > Options” and now: “Use PNGOUT PlugIn” > “Options” and there you have them all:
! If the plugin is not working get the full latest plugins package from the irfanView website and install it.
Here you can finally select all that can be set so that your PNG fits the specs.
How can I test my results?
Also a solution for this: pngcheck. It comes handy open source and for Mac, Linux and Win.
Switching to the console (Win example) you need the verbose mode “-v”:
C:\programs\pngcheck>pngcheck.exe -v C:\images\your_image.png
File: C:\images\your_image.png (731 bytes)
chunk IHDR at offset 0x0000c, length 13
250 x 250 image, 8-bit grayscale, non-interlaced
chunk IDAT at offset 0x00025, length 674
zlib: deflated, 32K window, fast compression
chunk IEND at offset 0x002d3, length 0
No errors detected in C:\images\your_image.png (3 chunks, 98.8% compression).
And this is, how you deliver a grayscale PNG free and open source.