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"

To use this, save the above script somewhere like ~/bin. Then, in the terminal:

chmod +x ~/bin/retinaify.sh
~/bin/retinaify.sh /path/to/file.png

Note that I have no idea whether all three of the above xattrs are required to make this work, but this script works and I have other things to do…

Also, the last line that opens the file in Preview (or your viewer of choice) is optional. Comment it out with a # char if you don't need it.

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