What about mixing both proposals:

    <picture alt="A giant stone" src="stone.jpg, stone@2x.jpg, stone@600w.jpg">
      <img src="stone.jpg" />
    </picture>

Inner content should not be rendered by clients supporting the picture tag. This allows for a natural fallback without using <noscript>, like we already have for canvas.

I'm not a great fan of "picture" either. We already have <figure>. Why not <image>?

The ideal solution IMO would be something like Progressive JPEG (with less artifacts): you serve a single high-quality image, and the browser downloads as much of it as it sees fit for the situation. Smaller images would be created from a large, lower quality download.