Prova cosi':
codice:
<select name="menusx_off" size="5" 
onChange="document.getElementById('URL_menusx_off').src = 'pulsanti/' + this.options[this.selectedIndex].value;">
<option value="menusx_bg_but_01.gif">Pulsante 1</option>
<option value="menusx_bg_but_02.gif">Pulsante 2</option>
<option value="menusx_bg_but_03.gif">Pulsante 3</option>
<option value="menusx_bg_but_04.gif">Pulsante 4</option>
<option value="menusx_bg_but_05.gif">Pulsante 5</option>
</select>
[img]pulsanti/menusx_bg_but_00.gif[/img]
ciao