求解!!请问怎么改变动态按钮的背景呢？

HTML code

 <div style="left: 20px; margin-left: 50px; width: 273px; height: 21px">                    <input id="S_btn" style="width: 38px; height: 23px; cursor: hand;" title="点击选择" type="button" value="S"/>                    <input id="M_btn" style="width: 37px; height: 23px; cursor: hand;" title="点击选择" type="button" value="M" />                    <input id="L_btn" style="width: 37px; height: 23px; cursor: hand;" title="点击选择" type="button" value="L" />                    <input id="XL_btn" style="width: 37px; height: 23px; cursor: hand;" title="点击选择" type="button" value="XL" />                </div>

只是HTML里面的4个按钮，现在我想点击按钮的时候，按钮的背景图片为***.gif；按钮的边缘为1PX的实线，颜色为#ff8080，该怎么做呢？
谢谢

[解决办法]

HTML code

    <div style="left: 20px; margin-left: 50px; width: 273px; height: 21px">        <input id="S_btn" style="width: 38px; height: 23px; cursor: hand;" title="点击选择" type="button"            value="S" onclick="setBackground(this)" />        <input id="M_btn" style="width: 37px; height: 23px; cursor: hand;" title="点击选择" type="button"            value="M" />        <input id="L_btn" style="width: 37px; height: 23px; cursor: hand;" title="点击选择" type="button"            value="L" />        <input id="XL_btn" style="width: 37px; height: 23px; cursor: hand;" title="点击选择"            type="button" value="XL" />    </div>    <script type="text/javascript">        function setBackground(o) {            o.style.border = 'dashed 1px #ff8080';        }    </script>