[Help]Didn't upload and Resize Image Codeigniter

For the http error 500 is done. but i have some problem. Why when i upload a file its not uploaded. And showing "Please check your image!" ?

This is for PHP using a framework Codeigniter

public function submit_image(){

    $input_name = $_POST['input_name'];
    $input_email = $_POST['input_email'];
    $input_code = $_POST['input_code'];

    $config['file_name'] = $input_code;
    $config['overwrite'] = TRUE;
    $config['upload_path']          = './img/';
    $config['allowed_types']        = 'gif|jpg|png|jpeg|pdf';
    $config['max_size']             = 5000000;
    //$config['max_width']            = 6000;
    //$config['max_height']           = 4000;

    $this->load->library('upload', $config);
    $this->upload->initialize($config);

  if(!empty($_FILES['filepicture']['name'])){

    if ($input_gambar = $this->upload->do_upload('doc')) {
    $data = $upload_data = $this->upload->data();

    //Compress Image


    $config['image_library'] = 'gd2';
    $config['source_image'] = './img/'.$data['file_name'];
    $config['create_thumb'] = FALSE;
    $config['maintain_ratio'] = FALSE;
    $config['quality'] = '50%';
    $config['width'] = 6000;
    $config['height'] = 4000;
    $config['new_image'] = './img/resized_'.$data['file_name'];
    $this->load->library('image_lib', $config);
    if (!$this->image_lib->resize()) {
        show_error($this->image_lib->display_errors());
    }
        echo "<script>alert('Thanks for upload Image');</script>";
        echo "<script>window.history.go(-2);</script>";            
    }else{
        echo "<script>alert('Please check your image!');window.history.go(-1);</script>";
    }
  }              
}