The server should not have to decide which image to send, based on whatever it can gather of a combination of various factors, including device display resolution capability (double-density vs regular pixel mapping) and connection quality (wireless vs broadband) or whatever else comes into play. The server should be advertising the capability of high res images and the user agent should be requesting them if it wants.
That's a great idea, but that's not currently how it works. All of the high-res website image stuff that I've seen all uses client-side JavaScript to fetch the 2x images.
