This plugin tells WordPress to create three additional sizes for images you upload. Then it outputs special HTML (via a shortcode) that works with the Picturefill library to achieve responsive images in content.
[responsive imageid="12" size1="0" size2="500" size3="1000"]
Make sure your current theme has
'add_theme_support( 'post-thumbnails' );'
in the functions.php file.
Here: http://css-tricks.com/hassle-free-responsive-images-for-wordpress