• Image upload form with 5 images
• And a controller function that will upload those
• Thumbnails will be there too

Ok, lets start with the HTML form code, it is very basic:
views/upload_form.php

<form method="post" action="uploader/go" enctype="multipart/form-data">
  <input type="file" name="image1" /><br />
  <input type="file" name="image2" /><br />
  <input type="file" name="image3" /><br />
  <input type="file" name="image4" /><br />
  <input type="file" name="image5" /><br />
  <input type="submit" name="go" value="Upload!!!" />
</form>

And now the controller:
controllers/Uploader.php

class Uploader extends Controller {
  function go() {
    if(isset($_POST['go'])) {
      /* Create the config for upload library */
      /* (pretty self-explanatory) */
      $config['upload_path'] = './assets/upload/'; /* NB! create this dir! */
      $config['allowed_types'] = 'gif|jpg|png|bmp|jpeg';
      $config['max_size']  = '0';
      $config['max_width']  = '0';
      $config['max_height']  = '0';
      /* Load the upload library */
      $this->load->library('upload', $config);

      /* Create the config for image library */
      /* (pretty self-explanatory) */
      $configThumb = array();
      $configThumb['image_library'] = 'gd2';
      $configThumb['source_image'] = '';
      $configThumb['create_thumb'] = TRUE;
      $configThumb['maintain_ratio'] = TRUE;
      /* Set the height and width or thumbs */
      /* Do not worry - CI is pretty smart in resizing */
      /* It will create the largest thumb that can fit in those dimensions */
      /* Thumbs will be saved in same upload dir but with a _thumb suffix */
      /* e.g. 'image.jpg' thumb would be called 'image_thumb.jpg' */
      $configThumb['width'] = 140;
      $configThumb['height'] = 210;
      /* Load the image library */
      $this->load->library('image_lib');

      /* We have 5 files to upload
       * If you want more - change the 6 below as needed
       */
      for($i = 1; $i < 6; $i++) {
        /* Handle the file upload */
        $upload = $this->upload->do_upload('image'.$i);
        /* File failed to upload - continue */
        if($upload === FALSE) continue;
        /* Get the data about the file */
        $data = $this->upload->data();

        $uploadedFiles[$i] = $data;
        /* If the file is an image - create a thumbnail */
        if($data['is_image'] == 1) {
          $configThumb['source_image'] = $data['full_path'];
          $this->image_lib->initialize($configThumb);
          $this->image_lib->resize();
        }
      }
    }
    /* And display the form again */
    $this->load->view('upload_form');
  }
}

And that is it. Customize it to your needs, the basics are there already. And good luck!