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

Tutorial

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