Specifying image dimensions is enought to prevent any jumps.
Went with a padding-top + absolute position in wrapper hack at the end. That jumpiness is part the tooling problem.
I haven't looked in details into the new srcset and size attributes of image, but won't that help also?
Check out http://scottjehl.github.io/picturefill/
You still have to know the aspect ratio of the original image of course, but it does help.
On a second thought, cw, ch as in container width, height would be a great addition.
Isn't that achieved by just defining height and width in percentages?
Small, medium, large?
And what kind of display would you like those for?
1x, 2x or maybe 3x?