1.- de esta manera: 
'modules' => [
    'gridview' =>  [
        'class' => '\kartik\grid\Module',
                    ],
     'redactor' => [
            'class' => '\yii\redactor\RedactorModule',
            'uploadDir' => '@webroot/path/to/uploadfolder',
            'uploadUrl' => '@web/path/to/uploadfolder',
            'imageAllowExtensions'=>['jpg','png','gif']
        ]
        ],
  
2.- O de esta otra manera:
  return [
    'id' => 'app-backend',
    'basePath' => dirname(__DIR__),
    'controllerNamespace' => 'backend\controllers',
    'bootstrap' => ['log'],
    'modules' => [ 'redactor' => 'yii\redactor\RedactorModule'], /* el redactor esta aqui */
    'components' => [
        'request' => [
            'csrfParam' => '_csrf-backend',
        ],
        'user' => [
            'identityClass' => 'common\models\User',
            'enableAutoLogin' => true,
            'identityCookie' => ['name' => '_identity-backend', 'httpOnly' => true],
            'authTimeout' => 60 * 10, // auth expire 
        ],
        'session' => [
            // this is the name of the session cookie used for login on the backend
            'name' => 'advanced-backend',
            'cookieParams' => [/*'httponly' => true,*/'lifetime' => 60 * 10//3600 * 24 * 30
            ],
            'timeout' => 60 * 10,//3600 * 24 * 30,
            'class' => 'yii\web\DbSession',
        ],
        'log' => [
            'traceLevel' => YII_DEBUG ? 3 : 0,
            'targets' => [
                [
                    'class' => 'yii\log\FileTarget',
                    'levels' => ['error', 'warning'],
                ],
            ],
        ],
        'errorHandler' => [
            'errorAction' => 'site/error',
        ],
        /*
        'urlManager' => [
            'enablePrettyUrl' => true,
            'showScriptName' => false,
            'rules' => [
            ],
        ],
        */
    'site/captcha/<refresh:\d+>' => 'site/captcha',
    'site/captcha/<v:\w+>' => 'site/captcha',
    ],
    'params' => $params,
];