According to [theme_image_style][1] page you tried wrong parameters, 

as an example, your code should be something like the following snippet

    $variables = array(
            'style_name' => 'thumbnail',
            'path' => $image_fields[LANGUAGE_NONE][0]['uri'],
            'width' => $image_fields[LANGUAGE_NONE][0]['width'],
            'height' => $image_fields[LANGUAGE_NONE][0]['height']
    );
    print theme_image_style($variables);

and in short


    print theme('image_style', array(
            'style_name' => 'thumbnail',
            'path' => $image_fields[LANGUAGE_NONE][0]['uri'],
    ));
  [1]: https://api.drupal.org/api/drupal/modules%21image%21image.module/function/theme_image_style/7.x