Php bandeau aléatoire

Bonjour,

Pourriez vous me dire pourquoi ce code ne marche pas svp ? :

-------------
<?php

mysql_connect("localhost", "root", "");
mysql_select_db("prd");

$reponse = mysql_query("SELECT imagepub FROM bandeaux WHERE numpub = $numbandeau");

$nbbandeaux = mysql_num_rows($reponse);

$numbandeau = rand(0,3);

while ($donnees = mysql_fetch_array($reponse))
{
echo '<img src="illus/'.$donnees['imagepub'].'"/>';
}

mysql_close();

?>
-----------
Messages d'erreurs :

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in D:\Plusrichedemain\Site\bandeaux.php on line 8

Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in D:\Plusrichedemain\Site\bandeaux.php on line 12

--> lignes en gras

Merci.