Mine

<script>
function change_buttons(el3) {
	var buttons = document.getElementsByTagName('img');
	for (var i = 0; i < buttons.length; i++) {
	buttons.item(i).src = "" + i + "_" + el3.value + ".gif";	
}

</script>

Img color: <select name="el3" onchange="change_buttons(this)">
<option value="green">Green</option>
<option value="red">Red</option>
<option value="yellow">Yellow</option>
</select>

<br>

<img src="1_white.gif"> <img src="2_white.gif"> <img src="3_white.gif">