Try out this,
My installation is at D:\xampp\htdocs\advanced
\Yii::$app->basePath
will give like D:\xampp\htdocs\advanced\backend
.
\Yii::$app->request->BaseUrl
will give like localhost\advanced\backend\web\
You may store the image using \Yii::$app->basePath
and show it using \Yii::$app->request->BaseUrl