One way to improve this would be to set a cookie and then on the next request serve _2x images. That way all second requests would perform much better and consume less bandwidth, and considering the sizes of this _2x images we can neglect the overhead of the cookie ...

Unless you actually do want to serve the low-res images first, to make the pages download and render faster initially. On 3G it probably makes a big difference.

I sure would like to see a (simple!) web standard for this. Maybe with something like a 2x_src or HDsrc addition to the img tag.