sudo apt-get install gimp graphicsmagick-libmagick-dev-compat libmagickwand-dev
convert cat.jpg -quantize GRAY -colors 256 grey.jpg
# Gimp -> Colors -> Desaturate
convert input.jpg -resize 50% output.jpg
# Gimp -> Image -> Scale Image
convert input.jpg -resize '150x150' -gravity center -crop '100x100+0-4' output.jpg
# Gimp -> Ctrl-A -> Ctrl-X -> Ctrl-N -> Ctrl-V -> Layer -> Scale Layer