Update Image from database using ajax, jquery,and PHP not working

I wanna update profile picture of user that has logged in to my website. I use ajax, jquery, php to update data, in order to update data without refresh the page. This code just working to upload image into folder, but when I use this code to update image from database, it only sent null into database.

jquery and ajax script

$("#form-ava").on('submit',(function(e) {
    e.preventDefault();
    $.ajax({
        url: "../config.inc/upload.php",
        type: "POST",
        data:  new FormData(this),
        contentType: false,
        cache: false,
        processData:false,
        beforeSend : function()
        {
            //$("#preview").fadeOut();
            $("#err").fadeOut();
        },
        success: function(data)
        {
            if(data=='invalid')
            {
                // invalid file format.
                $("#err").html("Invalid File !").fadeIn();
            }
            else
            {
                // view uploaded file.
                $("#preview-ava").html(data).fadeIn();
                $("#form-ava")[0].reset();
      $("#hidden-list").hide();
            }
        },
        error: function(e)
        {
            $("#err").html(e).fadeIn();
        }
   });
}));

And it's the php syntax upload.php

<?php
require_once 'koneksi.php';
 if($_POST)
{
  $id_user= $_POST['id_user'];
  $img = $_FILES['image']['name'];
  $tmp = $_FILES['image']['tmp_name'];

$valid_extensions = array('jpeg', 'jpg', 'png', 'gif', 'bmp'); // valid extensions
$path = '../images/ava/'; // upload directory
  try {

// get uploaded file's extension
$ext = strtolower(pathinfo($img, PATHINFO_EXTENSION));

// can upload same image using rand function
$final_image = rand(1000,1000000).$img;

// check's valid format
if(in_array($ext, $valid_extensions))
{
    $path = $path.strtolower($final_image);

    if(move_uploaded_file($tmp,$path))
    {
        echo "<img src='$path' />";

}
else
{
    echo 'invalid';
}
$update = $db_con->prepare("UPDATE tb_users SET image=:img WHERE id_user=:id_user");
$update->bindparam(":id_user", $id_user);
$update->bindparam(":img", $image);
$update->execute();
$count = $update->rowCount();
if($count>0) {
echo "success";
}else {
echo "can't update!";
}
}
}catch(PDOException $e) {
echo $e->getMessage();
}
}
?>

HTML syntax

<form id="form-ava" action="../config.inc/upload.php" method="post" enctype="multipart/form-data">
         <input type="hidden" id="id_user" name="id_user" value="<?php echo $row->id_user;?>">
                <input id="ava-img" type="file" name="image" value="<?php echo $row->image;?>"/>
                    <input id="button" type="submit" value="Simpan" name="update"></br>
         <a href="#" class="btn btn-large btn-success" id="cancel-act"></i> &nbsp; Batal</a>
     </form>
       <div id="err"></div>