WordPress function to create picturefill HTML.

functions.php

  function get_responsive_image($id, $src, $alt = "") {
	$large = wp_get_attachment_image_src( $id, 'large' );
	$medium = wp_get_attachment_image_src( $id, 'medium' );
	$small = wp_get_attachment_image_src( $id, 'thumbnail' );

	$output = '<div class="img-responsive">';
	$output.= '  <div data-picture data-alt="' . $alt . '">';
	$output.= '    <div data-src="' . $small[0] . '"></div>';
	$output.= '    <div data-src="' . $medium[0] . '" data-media="(min-width: 786px)"></div>';
	$output.= '    <div data-src="' . $large[0] . '" data-media="(min-width: 920px)"></div>';
	$output.= '    <div data-src="' . $src . '" data-media="(min-width: 1140px)"></div>';
	$output.= '    <noscript>';
	$output.= '      <img src="' . $small[0] . '" alt="' . $alt . '">';
	$output.= '    </noscript>';
	$output.= '  </div>';
	$output.= '</div>';

	return $output;
}