Dithering

With Imagemagick

This will create a create a dithered copy of the image.

Don't use JPG as the output format. GIF and PNG will result in smaller file sizes.
Use -colorspace gray to produce a grayscale image.

magick convert pictochat_love_letters.jpg -colorspace RGB -ordered-dither o8x8 pictochat_love_letters.dith.gif

If you have a folder of files you want to process, you can use:

find . -type f -exec magick convert {} -colorspace RGB -ordered-dither o8x8 -set filename:f '%t' '%[filename:f].png' \;