Example of using responsive-io with Wordpress featured images

rio-featured-images.php

<?php 

// Get our featured image contents
$thumb_id = get_post_thumbnail_id( $post->ID );
$thumb_src = wp_get_attachment_url( get_post_thumbnail_id($post->ID) );
$thumb_alt = get_post_meta($thumb_id, '_wp_attachment_image_alt', true);

// Make sure we do have an 'alt' attribute set
if(empty($thumb_alt)) $thumb_alt = get_the_title();

// Make it responsive-io ready
echo '<img data-src="'.$thumb_src.'" class="attachment-post-thumbnail wp-post-image" alt="'.$thumb_alt.'">';