the_content does not include images, preventing preg_replace

I am using the jQuery Image Lazy Loading Plugin on my WP 3.3.1 blog, however I have found that the_content does not include the image URLs, so the preg_replace that this function does cannot find any image URLs to replace.

function filter_the_content($content) {
    if (is_feed()) {
        return $content;
    }
    return preg_replace_callback('/(<\s*img[^>]+)(src\s*=\s*"[^"]+")([^>]+>)/i', array($this, 'preg_replace_callback'), $content);
}

What I am trying to establish is where does $content get passed in by (and why does it not contain the image URLs).

add_filter('the_content', array($this, 'filter_the_content'));

I have found this problem with other WP image lazy loading plugins, so it’s not localised to this particular plugin.

I am pulling my blog images from this code in single-blog.php:

<?php
while(the_repeater_field('blog_images')): 

        if(get_sub_field('blog_image_link'))
        {
            ?><a>"><img src="<?php the_sub_field('blog_image'); ?>"/></a><?php ;
        }else {
            ?><img src="<?php the_sub_field('blog_image'); ?>"/><?php ;
        }
endwhile;
?>

https://wordpress.org/plugins/jquery-image-lazy-loading/