This gist allows you to serve proper images according to the resolution of the window.

Retina goodie.js

/* 	Server side check if there is a cookie that defines 
	devicePixelRatio, if not, serve this javascript */ 

//Does it support it? no, default to 1;
if (window.devicePixelRatio === undefined) {
	window.devicePixelRatio = 1;
}

//Setup cookie date.
var now = new Date();
var time = now.getTime();
time += 10*24*365*3600 * 1000; //10 Years
now.setTime(time);

//Set the cookie
document.cookie = "devicePixelRatio="+window.devicePixelRatio+";path=/;expires="+now.toGMTString();

//Reload the page
window.location.reload();

/* 	Now you can use this cookie value to serve proper images
	to that device and avoid using more bandwidth */