webp-convert-directory

#!/bin/bash

PARAMS=('-m 6 -q 70 -mt -af -progress')

if [ $# -ne 0 ]; then
	PARAMS=$@;
fi

cd $(pwd)

shopt -s nullglob nocaseglob extglob

for FILE in *.@(jpg|jpeg|tif|tiff|png); do
    cwebp $PARAMS "$FILE" -o "${FILE%.*}".webp;
done

1. Install cwebp with brew install cwebp
2. Create this shell script and place it in /usr/local/bin/
3. Now you can use webp-convert-directory in any directory of images to create webp versions