how to specify img_url in captcha settings?

Hello everyone I'm having hard time with Codeigniter captcha library. Browser in my web page do not show any captcha img file, i think it is problem with img_url option because img file location is 'http://localhost/blog/captcha/1455115990.3884.jpg' and when i try directly access it throw browser, i just get codeigniter default 404 window, but in my captcha directory in my web project root folder codeigniter successfully creating captcha img files. Any suggestions what i can do differently?

Controller 

Code:

function post($postID){
   $this->load->model('comment');
   $data['comments']=$this->comment->get_comments($postID);
   $this->load->helper('form');

   $this->load->helper('captcha');

   $vals=array(
       'img_path'=> './captcha/',
       'img_url'=>base_url().'captcha/',
       'img_width'=>150,
       'img_height'=>30
   );
   $cap = create_captcha($vals);
   $this->session->set_userdata('captcha', $cap['word']);
   $data['captcha']=$cap['image'];

   $data['post']=$this->Post->get_post($postID);
   $this->load->view('post', $data);
}

View

Code:

<p>Captcha Code: <?=$captcha ?> <br /><?php
                   $data_form=array(
                       'name'=>'captcha'
                   );
               echo form_input($data_form);
               ?>

           </p>

Can you post a path where image file is saved? Also, do you use any rewrite rules?