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.

Usage

Shortcode - 
[responsive imageid="12" size1="0" size2="500" size3="1000"]

PHP (use in template files) -

<?php 
	$image = get_field('upload_image');
	$mappings = array(
        0 => 'small-img', // zero maps to default
        250 => 'large-img',
        1000 => 'full-width'
    );
?>
<span data-picture data-alt="<?php echo get_img_alt($image) ?>">
   	<?php echo getPictureSrcs($image, $mappings) ?>
   	<noscript> <?php echo wp_get_attachment_image($image, $mappings[2]) ?> </noscript>
</span>

Prereqs

Make sure your current theme has

'add_theme_support( 'post-thumbnails' );'

in the functions.php file.

Tutorial

Here: http://css-tricks.com/hassle-free-responsive-images-for-wordpress