[Hrimthurs]

Thanks!
I've tweaked that a bit and got it working perfectly.

<script language="javascript">
<!--
function ShowPic(sImage)
{
    // Get the select values
    var design = document.proof.design.options[document.proof.design.selectedIndex].value; 
    var style = document.proof.style.options[document.proof.style.selectedIndex].value; 
    var finish = document.proof.finish.options[document.proof.finish.selectedIndex].value; 
  
    // Create our Image
    var sImage = "images/" + design + style + finish + ".jpg";

    // Assign the image
   document.ShowRoom.src = sImage;
}
// -->
</script>
<form name="proof">
  <select name="design"  onChange="ShowPic()">
    <option value="1">Option 1</option>
    <option value="2">Option 2</option>
  </select>
  <select name="style" onChange="ShowPic()">
    <option value="a">Option a</option>
    <option value="b">Option b</option>
  </select>
  <select name="finish" onChange="ShowPic()">
    <option value="z">Option z</option>
    <option value="y">Option y</option>
  </select>
</form>
<img name="ShowRoom" src="images/1az.jpg">

Thanks for your help.