select下面option的选中下标selectedIndex

<img id='pic' src='http://oi5k7c9bd.bkt.clouddn.com/log_yuan.png'/>
<select id='sel' onchange="showImg()">
    <option value='http://oito4x28c.bkt.clouddn.com/mmexport1470584347994.jpg'>图片1</option>
    <option value='http://oito4x28c.bkt.clouddn.com/mmexport1470235530625.jpg'>图片2</option>
    <option value='http://oito4x28c.bkt.clouddn.com/mmexport1475076387668.jpg'>图片3</option>
</select>
<script>
  function showImg () {
    var oSelect = document.getElementById('sel');
    var index = oSelect.selectedIndex;   //option数组返回的同时会返回当前option的选中下标selectedIndex
    document.getElementById('pic').src = oSelect[index].value; 
  } 

</script>