image_style_url doesn't create images
From: https://drupal.org/drupal-7.20-release-notes

In addition, any code which programmatically generates a link to an
image derivative without using the standard image_style_url() API
function will no longer work correctly if the image does not already
exist in the file system, since the necessary token will not be
present in the URL.

Instead you can do something like this;
$image_uri      = $node-&gt;field_image[LANGUAGE_NONE][0]['uri']; // or any public://my_image
$style          = 'my_style';
$derivative_uri = image_style_url($style, $image_uri);
$success        = file_exists($derivative_uri) || image_style_create_derivative(image_style_load($style), $image_uri, $derivative_uri);

$new_image_url  = file_create_url($derivative_uri);