Responsive Images using adaptImages.js
The two images below look the same when seen in a browser with width greater than 640 pixels, but if you resize your browser or if you’re viewing this page on a device with a lesser width, you will notice that the second image is aware of it’s context and will show a version more appropriate for it’s size.
A plain old image
At small browser widths, when this image is simply scaled down, you can barely recognize Obama.

A responsive image that uses adaptImages.js
At small browser widths, a cropped version, more suited for that size is shown.
CreditsI borrowed the idea of using this particular image from Jason Grigsby, his blog posts on responsive images are quite informative.
Photo of Obama licensed under Creative Commons. CC BY-NC-SA 2.0 from BarackObamaDotCom