Check if page/post has any anchors that link to an image jpg/gif/png

This method has worked in the past for checking if a shortcode exists on a page. You will have to test it and see if it fires in time to enqueue the javascript>

add_filter('the_posts', 'c3_image_check'); 
function c3_image_check($posts){
    if (empty($posts)) return $posts;
    $img_found = false; 
    foreach ($posts as $post) {
        if (stripos($post->post_content, '<img src="https://wordpress.stackexchange.com/questions/53585/)) {
            $img_found = true; // bingo!
            break;
        }
    }
    if ($img_found) {
        wp_enqueue_script("jquery.fancybox', LIB .'/jquery.fancybox.min.js', array( 'jquery' ) );
    }

    return $posts;
}