想实现点击即展开,再点击即关闭的时候自动切换标题背景图片。请问如下js代码错在哪呢?没有实现切换背景图片的功能
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title></title>
<link href="images/css.css"  rev="stylesheet" rel="stylesheet"  type="text/css" />
</head>
<body>
<div id="left_boder">
<div id="left_title"><span id="title_word">选择</span></div>
        <div class="dropDownList">
        <div id="dropDownList1" class="dropdown">
            <select name="" style="display:none;">
                <option selected="selected">教师类别</option>
                <option>兼职</option>
                <option>在职</option>
             </select>
            <span>教师类别</span>
            <ul></ul>
        </div>
        <div id="dropDownList7" class="dropdown">
            <select name="" style="display:none;">
                <option selected="selected">11</option>
                <option>12</option>
            </select>
            <span>请选择</span>
            <ul></ul>
        </div>
    </div>
    <div id="left_img_margin"><img src="images/sheding.gif" border="0" onclick=""  /> <img src="images/qingchu.gif"  border="0" onclick=""  /></div>
</div>
</body>
</html>
<script type="text/javascript">
    var spans=document.getElementsByTagName("span");
var selects = document.getElementsByTagName('select');
for(var i=0;i<spans.length;i++)
{
var select = selects[i];
span[i].onclick=function()
{
for(var i=0;i<selects.length;i++)
{
this.select.style.display=='none'?this.select.style.display='':this.select.style.display='none';
if(this.select.style.display=='none')
{
this.style.backgroundImage="url(left_b_bg0.GIF)";
}
else
{
this.style.backgroundImage="url(left_b_bg.GIF)";
}
}
}
}
</script>
<script src="images/js.js" language="javascript1.1" type="text/javascript"></script>