I like the proposed
<picture> markup except for the tag name. “Picture” is much more specific than “image”, and I bet a very large portion of images used on the web are not pictures. “Photo” would be worse, but “picture” still implies a complete photo, illustration, or diagram, whereas “image” encompasses those plus patterns, textures, gradients, and every other use of image data in use on the web today.
How about using that proposed
<picture> markup but instead calling the top-level tag