Image uploading with ajax!

I am not sure what i am doing wrong with my code from controller! here is the code to upload image via ajax submission:

PHP Code:

public function post()
    {
        $formrules = array(
            array(
                'field' => 'title',
                'label' => 'News Link',
                'rules' => 'required|max_length[200]'
            ),
            array(
                'field' => 'news',
                'label' => 'News Snippet',
                'rules' => 'required|max_length[2500]'
            )
        );

        $this->form_validation->set_rules($formrules);

        if ($this->form_validation->run() == TRUE)
        {
            # Started working with file upload.
            $validFiles = array(
                'upload_path'   => 'tmp_data/',
                'allowed_types' => 'jpg|png|gif',
                'max_size'      => 250
            );
            $this->load->library('upload', $validFiles);
            if ($this->upload->do_upload('newsphoto'))
            {
                echo "Image Uploaded Successfully!";
                # Add data to db.
                $this->load->model('links');
                $data = array(
                    'title' => $this->input->post('title'),
                    'news'  => $this->input->post('news')
                );
                $this->links->add($data);
            }
            else
            {
                echo $this->upload->display_errors();
            }
            # End all the process of file upload.
        }
        else
        {
            echo validation_errors();
        }
    } 


it is a method of a controller, but every time i started to upload something it seems to be an error, a msg shows like this:

You did not select a file to upload.

any idea on what i am missing?
Thanks CI Community