我要让top.gif始终水平居中该怎样修改代码？（无论任何时候）
css样式表代码如下：
a{text-transform:none;text-decoration:none;color:#000;}
a:hover{color:#555;text-decoration:underline;}
body{
font-size:9pt;
font-family:宋体;
color:#000;
margin:0px;
background:url(top.gif) no-repeat;
scrollbar-face-color:#BCD4EB;
scrollbar-base-color:#BCD4EB;
scrollbar-3d-light-color:#000;
scrollbar-arrow-color:#000;
scrollbar-darkshadow-color:#000;
scrollbar-highlight-color:#fff;
scrollbar-shadow-color:#999;
scrollbar-track-color:#fff;
}
table{font-size:9pt;font-family:宋体;color:#000;border-collapse:collapse;}
input{border:1px;font-size:12px;}
form{padding:0px;margin:0px;}
#topmenu{border:1px #1B8CB8 solid;margin:0px 0px 5px 0px;text-align:center;
filter:progid:dximagetransform.microsoft.gradient(gradienttype=0, startcolorstr=#93BBDF, endcolorstr=#EAF1F9);}
#footmenu{border:1px #1B8CB8 solid;margin:5px 0px 5px 0px;text-align:center;
filter:progid:dximagetransform.microsoft.gradient(gradienttype=0, startcolorstr=#93BBDF, endcolorstr=#EAF1F9);}
.tml{}
.tmc{height:32px;}
.tmr{}
.fml{}
.fmc{height:32px;}
.fmr{}
#left{padding:0px;width:200px;}
#center{width:5px;}
#right{padding:0px;width:545px;}
#show{padding:0px;}
#foot{height:60px;width:740px;text-align:center;}
#gopic{overflow:hidden;width:540px;}
.1{display:none;}
.2{border:1px #1B8CB8 solid;width:205px;height:28px;
filter:progid:dximagetransform.microsoft.gradient(gradienttype=0, startcolorstr=#93BBDF, endcolorstr=#EAF1F9);}
.3{display:none;}
.4{display:none;}
.5{border-left:1px #1B8CB8 solid;border-right:1px #1B8CB8 solid;width:205px;background:#BCD4EB;}
.6{display:none;}
.7{display:none;}
.8{border-top:1px #1B8CB8 solid;width:205px;}
.9{display:none;}
.11{display:none;}
.22{border:1px #1B8CB8 solid;width:555px;height:28px;
filter:progid:dximagetransform.microsoft.gradient(gradienttype=0, startcolorstr=#93BBDF, endcolorstr=#EAF1F9);}
.33{display:none;}
.44{display:none;}
.55{border-left:1px #1B8CB8 solid;border-right:1px #1B8CB8 solid;width:555px;background:#BCD4EB;}
.66{display:none;}
.77{display:none;}
.88{border-top:1px #1B8CB8 solid;width:555px}
.99{display:none;}
.111{display:none;}
.222{border:1px #1B8CB8 solid;width:768px;height:28px;
filter:progid:dximagetransform.microsoft.gradient(gradienttype=0, startcolorstr=#93BBDF, endcolorstr=#EAF1F9);}
.333{display:none;}
.444{display:none;}
.555{border-left:1px #1B8CB8 solid;border-right:1px #1B8CB8 solid;width:768px;background:#BCD4EB;}
.666{display:none;}
.777{display:none;}
.888{border-top:1px #1B8CB8 solid;width:770px;}
.999{display:none;}
.left_title{margin:3px;padding:4px;border:#1B8CB8 solid 1px;background:url(bgbg.gif) #efefef;}
.right_title{margin:3px;padding:4px;border:#1B8CB8 solid 1px;background:url(bgbg.gif) #efefef;}
.show_title{margin:3px;padding:4px;border:#1B8CB8 solid 1px;background:url(bgbg.gif) #efefef;}
.left_content{margin:3px;padding:2px;border:#1B8CB8 solid 1px;background:#fff;}
.right_content{margin:3px;padding:2px;border:#1B8CB8 solid 1px;background:#fff;}
.show_content{margin:3px;padding:2px;border:#1B8CB8 solid 1px;background:#fff;}
.left_title_left{width:20px;background:url(left_title.gif) no-repeat;}
.left_title_right{width:120px;}
.right_title_left{width:20px;background:url(left_title.gif) no-repeat;}
.right_title_center{width:440px;}
.right_title_right{width:40px;}
.p{width:114px;height:90px;border:1px solid #999;}
.p1{border:#eee 1px solid;}
.p2{border:#ddd 1px solid;}
.p3{border:#999 1px solid;padding:4px;}
ul.list{padding-right: 0px; list-style: none none outside; padding-left: 0px; padding-bottom: 0px; margin: 0px; padding-top: 0px}
ul.list li {padding-left: 15px; background: url(li.gif) no-repeat left 7px; padding-top: 3px;height:19px}
.li_left{overflow:hidden;text-overflow:ellipsis;white-space:nowrap;width:280px;height:14px;}
.li_right{overflow:hidden;text-overflow:ellipsis;white-space:nowrap;width:100px;height:14px;}
.bg{background:#efefef;}
.br{height:5px;}
.input{border:solid 1px #1B8CB8;color: #000000; font-size: 12px;;background:#EAF1F9;}
.text{border:solid 1px #1B8CB8;color: #000000; font-size: 12px;;background:#EAF1F9;}
.put{
filter:progid:dximagetransform.microsoft.gradient(gradienttype=0, startcolorstr=#93BBDF, endcolorstr=#EAF1F9);
border:1px solid #1B8CB8;
font-size: 9pt;
width:54px;
cursor:hand;
padding-top:2px;
height: 20px;
}
.overput{
filter:progid:dximagetransform.microsoft.gradient(gradienttype=0, startcolorstr=#EAF1F9, endcolorstr=#93BBDF);
border:1px solid #1B8CB8;
font-size: 9pt;
width:54px;cursor:hand;
padding-top: 2px;
height:20px;
}
.overtab{
cursor:hand;
border:#1B8CB8 solid 1px;
background:#7EA86A;
padding:4px 5px 4px 5px;
width:70px;
margin:4px;
text-align:center;
filter:progid:dximagetransform.microsoft.gradient(gradienttype=0, startcolorstr=#EAF1F9, endcolorstr=#1B8CB8);
}
.tab{
filter:progid:dximagetransform.microsoft.gradient(gradienttype=0, startcolorstr=#93BBDF, endcolorstr=#EAF1F9);
cursor:hand;
border:#93BBDF solid 1px;
background:#7EA86A;
padding:4px 5px 4px 5px;
width:70px;
margin:4px;
text-align:center;
}