Problem with new formats is legacy support. Something that could degrade gracefully on browsers that don't add support would be very welcome. Maybe the best solution is to keep the current 'img' tag notation, but somehow indicate to browsers that double-rez assets are likely to be available at something like 'filename_x2.ext', and they should request those when they come across an img tag, instead of 'filename.ext', falling back with a second request for the latter in case the '_x2' isn't found. Something like:
<meta img-x2-res="_x2" />
Encoding the image twice with a standardized addition to the filename is just an extra bit to add to the export macro. The image storage is irrelevant for most cases (assuming you compress appropriately); it's the bandwidth and request volume that's precious.
If you are going to do that you may as well have a meta tag to set a suffix "to find the jpeg2000 version of the image", and browsers that understand that format can then get the better format.
