Can't use thumbnail() function. Why?

Question

Laravel

Here is the controller:

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use Illuminate\Foundation\Validation\ValidatesRequests;
use TCG\Voyager\Traits\Resizable;
use App\Blog;
use DB;

class BlogController extends Controller
{

  use ValidatesRequests;
  use Resizable;

  public function index()
  {
    $posts = DB::table('posts')->orderBy('created_at', 'desc')->paginate(5);
    return view('blog', compact('posts'));
  }
}

And this is how I render the image:

@foreach( $posts as $post )
            <img src="{{ Voyager::image($post->thumbnail('small')) }}" alt="">
@endforeach

The function itself is stored here -> \vendor\tcg\voyager\src\Traits\Resizable.php
I would be very grateful for the help!