:: YOU HAVE TO INSTALL IMAGEMAGICK ON WINDOWS TO RUN THIS SCRIPT. (SEE the .sh script for linux)
:: Source : https://site.sarahgarcin.com/web-frugal/#images
:: USE : 
:: 1- place the dither_nb.bat in a folder with your images.
:: 2- Make sure the .bat is executable.
:: 3- Double-click: It’s done !

for %%i in (*.jpg) do convert "%%i" -resize 400x400 -strip -quality 86 -colorspace Gray -dither FloydSteinberg -colors 4 "dither_%%i"