[CSS3] The picture element

<picture>
  <source media="(min-width: 1000px)" srcset="kookaburra_large_1x.jpg 1x, kookaburra_large_2x.jpg 2x">
  <source media="(min-width: 500px)" srcset="kookaburra_medium_1x.jpg 1x, kookaburra_medium_2x.jpg 2x">
  <img src="kookaburra_small.jpg" alt="The kookaburra: a terrestrial tree kingfisher native to Australia and New Guinea">
</picture>

 http://udacity.github.io/responsive-images/examples/3-08/pictureFullMonty/