Don't retinafy at all. You should not assume your user has LTE speeds on his shiny iPad. As long as webdevs won't have a better navigator.connection, retinafying should be avoided.
On the web, loading times trump beauty.
On the web, loading times trump beauty.
If you've figured out how to display the same non-vector images at full quality at arbitrary file dimensions with the same filesize, then you're going to be very, very rich.