Run this script in the wp-content/uploads directory, first making sure you have installed jpegoptim and pngcrush. change 1920> to 3.840> if you worry about @2x retina images

resize-images-to-1920px-wide.sh

#!/bin/bash


for f in $(find . -name '*.jpg' -or -name '*.JPG' -or -name '*.JPEG' -or -name '*.jpeg' -or -name '*.png'); do 

convert "$f" -resize "1920>"  "$f";




filename=$(basename "$f")
extension="${filename##*.}"

echo $extension;

if [ $extension = "png" ]; then

#echo "is a PNG";

pngcrush -ow -rem alla -reduce "$f";

else

convert "$f" -quality 70 "$f";

jpegoptim "$f";

fi
done;