...
<script language="JavaScript" type="text/javascript" >
function setBeerIcon() {
 var beerIcon = document.getElementById("beerIcon");
 beerIcon.src = "images/"+getSelectValue("beer")+".jpg";
}
</script>
...
<img border="0" src="" id="brandIcon" alt="brand" />
<select name="beer" id="beer" onChange="setButton();setBeerIcon();">
<option value="--Select--">Select beer</option>
  <option value="heineken">heineken</option>
  <option value="sol">sol</option>
  <option value="amstellight">amstellight</option>
  <option value="coronalight">coronalight</option>
  <option value="coronaextra">coronaextra</option>
  <option value=""></option>
</select>