Ajax znovu načtení scriptu po zmáčknutí submitu

//... spojení s db ...
echo "<img src=\"".$promena['obrazek']."\" />";

<script src="//ajax.googleapis.com/ajax/libs/jquery/2.0.0/jquery.min.js"></script>
<script>
 
    function updateDb(valobr) {   
        var datastring = 'obrazek='+ valobr;
        $.ajax({
        type: "POST",
        url: "zmena_obr.php",
        data: datastring
        });
 
        return false;
 
    }
 
</script>

<form method="post">
    <input type="submit" name="zmena" value="Zelená" onclick="return updateDb('zelena.jpg')"/>
    <input type="submit" name="dolu" value="Modrá" onclick="return updateDb('modra.jpg')"/>
    
</form>

<?php
 
$heslo  = "vertrigo";        
$server  = "localhost";
$login = "root";
$database = "system";
 
if (MySQL_Connect($server, $login, $heslo) && MySQL_Select_DB($database))
{ 
  
    if(!empty($_POST['obrazek']))
    {
        mysql_query("update zkouska SET obrazek= '".$_POST['obrazek']."' where type = 'obr'");
    }
 
}

<script>
          Casovac();

            function NewHodnota() {
      var xmlhttp;
            var    hodnotalist = document.getElementById('hodnota_obr');
                if (window.XMLHttpRequest) xmlhttp = new XMLHttpRequest();
                else xmlhttpp=new ActiveXObject("Microsoft.XMLHTTP"); 
                xmlhttp.onreadystatechange=function() {
                  if (xmlhttp.readyState==4 && xmlhttp.status==200) {
                      hodnotalist.innerHTML = xmlhttp.responseText;
                }    
              }    
                xmlhttp.open("GET", "obrazek.php", true);
                xmlhttp.send(); 
              Casovac();
            }
    
            function Casovac() { 
                 setTimeout("NewHodnota()", 1000);
            }
</script>
<span id='hodnota_obr'></span>