单一图片切换jquery

<script type="text/javascript" src="jquery-1.8.3.min.js"></script>
?? ??? ?<style>
?? ??? ??? ?.for{overflow: hidden; width: 100%; zoom: 1;}
?? ??? ??? ?.for ul{list-style: none;}
?? ??? ??? ?.for ul li{ float: left; margin: 5px;}
?? ??? ??? ?.for ul li img{display: block;}
?? ??? ??? ?.btn{ width: 100px; height: 45px; text-align: center; line-height: 45px; background: blue; color: #fff;
?? ??? ??? ??? ?-webkit-user-select:none;

?? -moz-user-select:none;

?? -ms-user-select:none;

?? user-select:none;
?? ??? ??? ?}
?? ??? ?</style>

<div class="for">?? ??? ??? ?<ul>?? ??? ??? ??? ?<li><img id="m1"? src="images/prt_1.jpg"/></li>?? ??? ??? ??? ?<li><img id="m2"? src="images/prt_1.jpg"/></li>?? ??? ??? ?</ul>?? ??? ??? ??? ?</div>?? ??? ?<div class="btn" onclick="forTo()">换菜按钮</div>?? ?</body>?? ?<script>?? ??? ?var m1="images/prt_1.jpg"?? ????? var m2="images/prt_2.jpg"?? ?????? var m3="images/prt_3.jpg"?? ??????? var m4="images/prt_4.jpg"?? ??????? var i=0?? ??????? var a = [m1,m2,m3,m4]// jquery型?? ??? ?$("#m1").click(function(){??????????? i++;??????????? if(i>a.length){??????????????? i=1;??????????? }??????????? $(this).prop("src","images/prt_"+i+".jpg");??????? });?? // 方法型?????? ??? ?function forTo(){??????????? i++;??????????? if(i>a.length){??????????????? i=1;??????????? }??????????? $(‘#m2‘).prop("src","images/prt_"+i+".jpg");??????? };?? ?</script>