For iPhone devs: Create a halved version of this file, and rename the full-size version to @2x.

convert.sh

#!/bin/bash

FILE=$1
BASE=${FILE%.*}
EXT=${FILE##*.}
cp "$FILE" "${BASE}@2x.${EXT}"
WIDTH_RAW=`sips -g pixelWidth "$FILE" | sed -n 2p`
WIDTH=${WIDTH_RAW##*:}
HEIGHT_RAW=`sips -g pixelHeight "$FILE" | sed -n 2p`
HEIGHT=${HEIGHT_RAW##*:}
HALF_WIDTH=`ruby -e "puts $WIDTH/2"`
HALF_HEIGHT=`ruby -e "puts $HEIGHT/2"`
sips -z $HALF_HEIGHT $HALF_WIDTH "$FILE"

# Only works on Macs since it relies on sips
# 
# Run like:
# ./convert.sh myimage.jpg
# Or to convert all .jpg's in a directory:
# ls -1 | grep .jpg | xargs -L 1 -J % ./convert.sh "%"