新手请问JS的一个事件对象有关问题_Java Web开发

新手请教JS的一个事件对象问题。
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<script type="text/javascript" src="js/jquery.js"></script>
<script type="text/javascript">
var fudu = null;
var fuhao = null;
$(function(){
	fudu = parseFloat($("#moren").val());
	$("#select").change(function(){
		fudu = parseFloat($("#select").val());
	});
	$(".quanbu .jia,.quanbu .jian").click(function(){
		fuhao = $(this).text();
		changeVal();
		// 如果是全部修改的就提交到 quanbu.php
		$.post("/index.php?r=admin/odds/quanbu",{"jiajian":fuhao,"fudu":fudu});
	});
	
	$(".d_jia,.d_jian").click(function(){
		fuhao = $(this).text();
		var suoyin = $($(".all_cont").find("tr:gt(0)")).index($(this).parents("tr:first"));
		switch(fuhao){
			case "+":
				$(this).siblings(".text").text(returnFloat2(parseFloat($(this).siblings(".text").text())+fudu));
				break;
			case "-":
				$(this).siblings(".text").text(returnFloat2(parseFloat($(this).siblings(".text").text())-fudu));
				break;
		}
		// 如果是单独修改就提交到 dandu.php
		$.post("/index.php?r=admin/odds/dandu",{"jiajian":fuhao,"fudu":fudu,"suoyin":suoyin},function(data){alert(data)});
	});
	
	
});
function changeVal(){
	$(".all_cont").find("tr:gt(0)").each(function(){
		switch(fuhao){
			case "+":
				$(this).find("td.num_c .text").text(returnFloat2(parseFloat($(this).find("td.num_c .text").text())+fudu));
				break;
			case "-":
				$(this).find("td.num_c .text").text(returnFloat2(parseFloat($(this).find("td.num_c .text").text())-fudu));
				break;
		}
	});
}

function returnFloat2(value) {
	value = Math.round(parseFloat(value) * 100) / 100;
	var xsd = value.toString().split(".");
	if(xsd.length==1){
		value = value.toString()+".00";
		return value;
	}
	if(xsd.length>1){
		if(xsd[1].length<2){
			value = value.toString()+"0";  
		}
		return value;
	}
} 
</script>
</head>
<body>
<table width="100%" border="0" cellpadding="0" cellspacing="1" class="all_cont">
	<tr>
		<th width="100"><strong>序号</strong></th>
		<th><strong class="quanbu"><span class="jia" style="cursor:pointer;">+</span> 利息 <span class="jian" style="cursor:pointer;">-</span></strong></th>
		<th width="100"><strong>实际</strong></th>
		<th><strong>自定义调节幅度</strong>
			<select name="fudu" id="select">
				<option value="1">1</option>
				<option value="2">2</option>
				<option value="5">5</option>
				<option value="10">10</option>
				<option value="20">20</option>
				<option selected="selected" value="0.5" id="moren">默认</option>
			</select>
		</th>
	</tr>
	<tr>
		<td align="center">1</td>
		<td align="center" class="num_c"><span class="d_jia" style="cursor:pointer;">+</span> <span class="text">99.50</span> <span class="d_jian" style="cursor:pointer;">-</span></td>
		<td align="center">111</td>
	</tr>
	<tr>
		<td align="center">2</td>
		<td align="center" class="num_c"><span class="d_jia" style="cursor:pointer;">+</span> <span class="text">88.50</span> <span class="d_jian" style="cursor:pointer;">-</span></td>
		<td align="center">111</td>
	</tr>
	<tr>
		<td align="center">3</td>
		<td align="center" class="num_c"><span class="d_jia" style="cursor:pointer;">+</span> <span class="text">77.50</span> <span class="d_jian" style="cursor:pointer;">-</span></td>
		<td align="center">111</td>
	</tr>
	<tr>
		<td align="center">4</td>
		<td align="center" class="num_c"><span class="d_jia" style="cursor:pointer;">+</span> <span class="text">66.50</span> <span class="d_jian" style="cursor:pointer;">-</span></td>
		<td align="center">111</td>
	</tr>
	<tr>
		<td align="center">5</td>
		<td align="center" class="num_c"><span class="d_jia" style="cursor:pointer;">+</span> <span class="text">55.50</span> <span class="d_jian" style="cursor:pointer;">-</span></td>
		<td align="center">111</td>
	</tr>
</table>
</body>
</html>
这样的一段代码中是正常的，效果是点击利息旁边的加号或者减号，下面的数字会有相应的变化。

但是目前我觉得那个加号和减号太难看了，我想换成IMG图片方式，意思是把
<th>+ 利息 -</th>