为什么我的背景没变色
<body>
<h3>修改表单中图像元素的背景色</h3>
<form id="frmTest" action="#">
<input type="image" src="http://img1.sycdn.imooc.com//52b284ea00016b2902590070.jpg" /><br />
<br />
<img alt="" src="http://img1.sycdn.imooc.com//52b284ea00016b2902590070.jpg" /><br />
</form>
<script type="text/javascript">
$("#frmTest :image").addClass("bg_red");
</script>
</body>
