How to modify the image output for just one content type?
You can do this by defining a "lazy load" image style and using it as the image display.  This style does not have to do any resizing.  It would just be a placeholder flag for a preprocess function like:

function my_theme_preprocess_image(&amp;$variables) {
  if ( $variables['style_name'] == 'lazy_load') {
    $variables['attributes']['data-src'] = $variables['attributes']['src'];
    unset($variables['attributes']['src']);
  }
}


Nice thing is that you can use this style anywhere to get lazy loading images.