A little bash script that takes a bunch of retina sized images, moves them to image@2x.png and then creates a half size version using imagemagick.

retina_resizer.sh

#!/bin/bash
# loops through a directory of files, moves all .png files to file@2x.png
# and then resizes it using imagemagick to 50% size in file.png

shopt -s nullglob # if there are no .png files in the directory, then the below line won't just return *.png
ext=png
for file in *.$ext
do
  text="${file##*.}"
  name="${file%.*}"
  new=$name@2x.$ext

  if ["$text"="$ext"]
  then
    mv $file $new
    convert $new -resize 50% $file
  fi
done