You’ve probably seen this already, but it’s really worth bearing in mind: when you’re scaling up JPGs for retina display you can safely reduce the image quality by quite a lot—to the point of getting the exact same file size as a higher quality image that’s half the size.
This looks like a really handy tool for reducing the file size of JPEGs without any perceptible loss of quality (in much the same way that ImageOptim works for PNGs)—available as a Mac app or an installable web service.
Here’s a brainbuster for ya: a single file that renders both as HTML and as a JPEG. As an HTML page, it even contains an img element with a src of …itself!
Compare the “view source” output with the generated source output to see it’s being interpreted.
An idea for handling responsive images not with a new format, but with an existing one: progressive JPGs.
Here’s a great braindump from Paul following the Responsive Summit, detailing multiple ways of potentially tackling the issue of responsive images.