点图片上一页下一页翻页效果

<script language="javascript" defer>
var zhang=2
function next(){
if(zhang==7){
alert("这已经是最后一张了!")
zhang=6
}
document.getelementbyid("tu").src=zhang+".jpg"
text.innerhtml="当前是第"+zhang+"张图片"
zhang++
}
function up(){
if(zhang==2){
alert("这已经是第一张了!")
zhang=3
}
document.getelementbyid("tu").src=(zhang-2)+".jpg"
text.innerhtml="当前是第"+(zhang-2)+"张图片"
zhang--
}
</script>

<style type="text/css">
#divall {
position:relative;
}
#divleft {
border:1px red solid;
border-right:0px red solid;
background: url(bg.gif);
cursor:url("2.cur");
position:absolute;
top:0px;
z-index:2007;
text-align:right;
padding:0px
}
#divright {
border:1px red solid;
border-left:0px red solid;
background: url(bg.gif);
cursor:url("1.cur");
top:0px;
position:absolute;
z-index:2007
text-align:center;
padding:0px
}
#tu{z-index:-2007}
</style>

<div id="text"  >当前是第1张图片</div>
<div id="divall">
<img src="1.jpg" id="tu">

<div id="divleft" title="上一张" onmouseover="show_div('divleft','left.gif')" onmouseout="hide_div('divleft')" onclick="up()">
</div>

<div id="divright" title="下一张"  onmouseover="show_div('divright','right.gif')" onmouseout="hide_div('divright')" onclick="next()">
</div>
</div>
<script>//

var w=document.getelementbyid("tu").width//500
var h=document.getelementbyid("tu").height//400
document.getelementbyid("tu").style.width=w
document.getelementbyid("tu").style.height=h

//document.getelementbyid("divleft").style.visibility='hidden'
document.getelementbyid("divleft").style.width=w/2
document.getelementbyid("divleft").style.height=h
document.getelementbyid("divleft").style.left=0

//document.getelementbyid("divright").style.visibility='hidden'
document.getelementbyid("divright").style.width=w/2
document.getelementbyid("divright").style.height=h
document.getelementbyid("divright").style.left=w/2

//document.write("<style> #tu{width:"+w+"px; height:"+h+"px;z-index:2000}</style>")

function show_div(id,img){
document.getelementbyid(id).innerhtml="<img src="+img+">"
}
function hide_div(id){
document.getelementbyid(id).innerhtml=""
}
</script>