设置iTop和iLeft属性，如下：
function openTempleteSelector(url,iWidth,iHeight){
var iTop = (window.screen.availHeight - 20 - iHeight) / 2;
var iLeft = (window.screen.availWidth - 10 - iWidth) / 2;
window.showModalDialog(url,window,"dialogWidth:"+iWidth+"px;dialogHeight:"+iHeight+"px;dialogTop:"+iTop+"px;dialogLeft:"+iLeft+"px;edge: Raised; center: Yes; help: No; resizable: Yes; status: No;");
onCharsChange();
}