[magick-users] Convert Image to Greyscale

[duc.sequere.aut.de.via.decede at imagemagick.org]

> results within [0..255].

Divide by 257 to convert 16-bit values to 8-bit.  You can also write to a
blob with a map of "RGB" and type of Char but that seems wasteful in memory
usage and processing time.