This script finds all the images in a folder (and its subfolders) that are larger than 3MB, resizes them to have a maximum size of 3000px on the longest side and replaces the original image file.

batch-resize-mac.sh

#!/bin/bash
# This script finds all the images in a folder (and its subfolders) that are larger than 3MB,
# resizes them to have a maximum size of 3000px on the longest side and replaces the original image file.
# Usage: > sh ./batch-resize-mac.sh ./path/to/folder

# Info: https://ss64.com/osx/sips.html
# Credit: https://gist.github.com/lopespm/893f323a04fcc59466d7

initial_folder="$1"
all_images=$(find -E $initial_folder -iregex ".*\.(jpg|gif|png|jpeg)")

max_size=3000000

while read -r image_full_path; do
  if [ ! -z "$image_full_path" -a "$image_full_path" != " " ]; then
      file_size=$(wc -c "$image_full_path" | awk '{print $1}')
      if (( file_size > max_size)); then
        sips -Z 3000 "$image_full_path" --out "$image_full_path";
      fi
  fi

done <<< "$all_images"