Chrome now supports srcset, which lets you write things like:

  <img src="image-1x.jpg"
          srcset="image-1x.jpg 1x,
                  image-2x.jpg 2x">

That is, you can now send high PI ("retina") images only to browsers that support them. Previously there weren't really good options for this, and you needed to do something dynamic based on the user-agent string, send the high PI image to all devices, do something in javascript, or worse. Now the browser can figure this out.

I've tweaked my blog software to put in a srcset when it sees an image with src foo.jpg and foo-2x.jpg is present on disk. [1] If your browser supports srcset and you have a screen with extra dots per inch, you should see "2x" overlaid on the image below and it should look extra sharp. Otherwise you should see "1x".

Really this is something PageSpeed should be able to do automatically...

[1] Actually, it's a little more complicated, because I've used a few conventions over the years for naming files. It actually looks for foo-2x.EXT if it sees any of foo.EXT, foo-sm.EXT, foo-small.EXT, or foo-tn.EXT. This is a case where a little more consistency would have helped, though it could have been worse.