Drupal cannot generate the derived image ANYMORE?
In your settings.php file add this

$conf['image_allow_insecure_derivatives'] = TRUE;


In the future you should use image_style_url() when using image styles as this will then add in itok parameter for images. Usage: image_style_url doesn&#39;t create images

More info about this: https://drupal.org/drupal-7.21-release-notes