Ho aggiunto qualche apostrofo, due punti e virgola ... (Totò e Peppino):
codice:
<form action="query.php?action=new_photo" method="POST" 

 onChange='alert("gest-photo.php?photo=" + photo.value + ""); 
           location.assign("gest-photo.php?photo=" + photo.value + "");'
>
<select name="photo">
 <option value="">- scegli chi -
 <option value="ortolani">Mario
 <option value="regnanti">Enzo
 <option value="sartori" >Dina
</select>
</form>