codice:
<SCRIPT> 
function changeText(elem){
selection = elem.taglia.options[elem.taglia.selectedIndex].value; 	document.frm.Submit.disabled=false; 
}
</SCRIPT>
<form name="frm" method="post"> 
<SELECT SIZE="1" NAME="taglia" onChange="changeImage(this)">
<option value="0">Seleziona taglia</option>
<option value="1">1</option>
<option value="2">2</option>
</select>
<input name="Submit" type="image" onclick="Invia()" src="butt_acquista.jpg">
</form>