Create SD, Retina and @3x Images of a given size.

scaleToWidth

#!/bin/sh

filename=$(basename "$1")
extension="${filename##*.}"
filename="${filename%.*}"

if [ -z $1 ]; then
    echo "Usage: $0 image.png <points wide>"
    echo "   produces 3 copies of the image, in SD, @2x and @3x for that point size"
    exit;
fi

src=$1
w=$2

sips -s format png --resampleWidth $w $src --out "${filename}.png"

sips -s format png --resampleWidth $(($w * 2)) $src --out "${filename}@2x.png"

sips -s format png --resampleWidth $(($w * 3)) $src --out "${filename}@3x.png"

More here: http://lifehacker.com/5962420/batch-resize-images-quickly-in-the-os-x-terminal

Note that the script also converts the files to PNG.