The simplest answer is just to double the dimensions of the image then use CSS to make it the right size.<p>So if you want to create an HD image at 200 x 200 on a Retina display, create the image at 400 x 400 then use CSS to control the HD image down to 200 x 200: img { width:200px; height:200px; }.<p>The HD images are still 72 dpi resolution.<p>The tricky part is then preventing non-HD devices from unnecessarily downloading the larger images.