Jason Grigsby knows a lot about the responsive web and has the wonderful ability to break down problems into simple questions. He does just this in this article on responsive images and looks at the issue of providing the right sized image and the correct one when it comes to art direction.
When the image is displayed at larger sizes, it makes sense for the image to show the automobile factory in the background. The background helps explain where the event took place and adds to the image. But look what happens when we scale the image down to fit a smaller screen.An excerpt from A framework for discussing responsive images solutions