为什么php回显图像只显示图像的缩略图？

MY PHP CODE IS BELOW echoed image shows only its thumbnail

<?php 
    if(isset($_SESSION['user_id'])){
        $userid =  $_SESSION['user_id'] ;
        $query = "SELECT img FROM user where iduser='{$userid}' LIMIT 1 ";
        $result_set = mysqli_query($connection,$query);
        while($row = mysqli_fetch_assoc($result_set)){
            echo $row['img'];
            if(is_null($row['img'])){
            echo '<img  src="assets/images/fuser.png" class="img-responsive img-thumbnail" style="height:256px;width:256px">';
            }else{
            echo '<img src="assets/images/"'.$row['img'].'" class="img-responsive img-thumbnail" style="height:256px;width:256px">';
            }
        }
    }
?>

My image shows like this https://imgur.com/a/uXcdytz

The html you're echoing is ill-formed:

echo '<img src="assets/images/"'.$row['img'].'" class="img-responsive img-thumbnail" style="height:256px;width:256px">';

For $row['img'] that's image1.jpg, your code will echo this:

<img src="assets/images/"image1.jpg" class="img-responsive img-thumbnail" style="height:256px;width:256px">

Which has an additional unnecessary quote in the src attribute, so the image src destination won't exist, your echo should be (after removing the " after images/):

echo '<img src="assets/images/'.$row['img'].'" class="img-responsive img-thumbnail" style="height:256px;width:256px">';

Also make sure that your php file is in the folder assets/.