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$uri = $rec->field_product_image['und'][0]['uri'];  
$imgpath =   image_style_path('style_1000x400', $uri);
if (!file_exists(drupal_realpath($imgpath))) {
   $styles = image_styles();
   image_style_create_derivative($styles['style_1000x400'], $uri, $imgpath);
}
$imgpath = file_create_url($imgpath);       
$tmp['uri'] = $imgpath;