Make Preview open your @2x .png files as if they are retina images

retinaify.sh

#!/bin/sh

# First, set the width/height DPI on the file
sips -s dpiWidth 144 -s dpiHeight 144 "$1"

# Fake that it is a screen capture (wat?)
xattr -wx com.apple.metadata:kMDItemIsScreenCapture "62 70 6C 69 73 74 30 30 09 08 00 00 00 00 00 00 01 01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 09" "$1"
xattr -wx com.apple.metadata:kMDItemScreenCaptureType "62 70 6C 69 73 74 30 30 59 73 65 6C 65 63 74 69 6F 6E 08 00 00 00 00 00 00 01 01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 12" "$1"
xattr -wx com.apple.FinderInfo "00 00 00 00 00 00 00 00 00 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00" "$1"

# Force spotlight to re-index the file to see the DPI change, etc.
mdimport "$1"

# Now, open it in preview
open "$1"

The com.apple.FinderInfo attr doesn't seem likely to be required for this one.