How to convert an image to Portable Pixel Map format:

GIF:

colour:
   giftopnm foo.gif > foo.ppm
greyscale: 
   giftopnm foo.gif > foo.pgm

JPG:

colour:
   jpgtopnm foo.jpg > foo.ppm
greyscale:
   jpgtopnm foo.jpg > foo.pgm
   convert to greyscale by opening in gimp and saving as 'raw pgm'
   (you will be asked if you wish to export: answer 'yes')

giftopnm and jpgtopnm are Linux utilities (in the default distribution).

Challenge: Does anyone know of a less awkward way to translate greyscale JPG?