You can't scale it on the client side, because one of the core reasons for doing this is to NOT download large images for areas were a mobile connectivity is a factor. So you can't scale it down unless you've already downloaded the large version. This wouldn't work.
You have to ONLY download the correct size you need.
This could be done manually with tags, which seems to be the chosen route. However I think what "bobbied" suggested above would be a good automated way of doing it. Say you set up a meta tag using the media queries in the HTML to be "mobile conscious". Then anytime it ran across an , the browser would request images as such: "GET image.jpg?max-width=18em" (or however else the developer defined the meta tags). The web server (which would also need to be aware of this type of request), would then return back a certain sized image. This would require the least amount of effort by the developers and would just need support by the browsers and web servers.