急求帮忙解决选择头像的问题

相对应的图片为 1.gif  2.gif  3.gif

<script language=javascript>
function check()
{
var s1=document.getElementById("s1");
var img=document.getElementById("img");
img.src=s1.value+".gif";
}
</script>
<img id="img" src="" width="80" height="100">
<select name="s1" onchange="check();">
<option value="0">请选择头像</option>
<option value="1">头像1</option>
<option value="2">头像2</option>
<option value="3">头像3</option>
</select>