This is a quick script to resize all JPEGs in a folder recursively and output them to another folder. It will only resize new images on subsequent runs to save time. The settings in the example are to max width 1280px and file size 200KB.

1. Install brew and dependencies:

brew install imagemagick jpegoptim

2. Download or copy and paste the following script resize-pics.sh:

   #!/bin/bash
   
   set -o nounset
   set -o errexit
   
   DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
   
   cd "$DIR"
   
   export SOURCE_DIR="Pics"
   export TARGET_DIR="Pics Resized"
   
   mkdir -p "$TARGET_DIR"
   
   function resizeimage {
   
       IMAGE_PATH="${1#*/}"
       OUTPUT_PATH="${TARGET_DIR}/${IMAGE_PATH}"
   
       if [[ "$OUTPUT_PATH" =~ '/' ]]
       then
           mkdir -p "$(dirname "$OUTPUT_PATH")"
       fi
   
       if [ ! -e "${OUTPUT_PATH}" ]
       then
           echo "Resizing ${IMAGE_PATH}"
           convert -resize '1280x>' "${SOURCE_DIR}/${IMAGE_PATH}" "$OUTPUT_PATH"
           jpegoptim -S200 "$OUTPUT_PATH"
       fi
   }
   
   export -f resizeimage
   
   find "$SOURCE_DIR" -type f \( -iname '*.jpg' -o -iname '*.jpeg' \) -exec bash -c 'resizeimage "$@"' bash {} \;
   
   read -rp "Done. Press Enter to exit"

3. Customize SOURCE_DIR (default Pics) and TARGET_DIR (default Pics Resized) in the script.

4. Make the script executable:

chmod u+x resize-pics.sh

5. Either run the script from Terminal as ./resize-pics.sh or set it up to be double-clickable in Finder (see https://stackoverflow.com/questions/5125907/how-to-run-a-shell-script-in-os-x-by-double-clicking)