Alumnos.php

<?php
defined("BASEPATH") or exit("No direct script access allowed");

class Alumnos extends CI_Controller
{
    public function ponerFotoAlumno()
    {
        $config['upload_path'] = './fotos_alumnos/';
        $config['allowed_types'] = 'gif|jpg|png';
        $config['max_size'] = 2048;

        $this->load->library('upload', $config);
        if (!$this->upload->do_upload('foto')) { #Aquí me refiero a "foto", el nombre que pusimos en FormData
            $error = array('error' => $this->upload->display_errors());
            echo json_encode($error);
        } else {
            echo json_encode(true);
        }

    }
}