window.onload=function(){
    preparegallery();
    showpic();
}
function showpic(whichpic){
    if (!document.getElementById("placeh")) return false;
    var source = whichpic.getAttribute("href");
    var placeh = document.getElementById("placeh");
    if (placeh.nodeName != "IMG") return false;
    placeh.setAttribute("src", source);
    if (document.getElementById("description"))
    {
        var text = whichpic.getAttribute("title") ?  whichpic.getAttribute("title"):"";
        var description = document.getElementById("description");
        if(description.firstChild.nodeType == 3)
        {
            description.firstChild.nodeValue = text;
        }
    }
    return true;
}

function preparegallery(){
    if(!document.getElementsByTagName) return falae;
    if(!document.getElementById) return false;
    if(!document.getElementById("imagegallery")) return false;
    var gallery=document.getElementById("imagegallery");
    var links=gallery.getElementsByTagName("a");
    for(var i=0;i<links.length;i++){
        links[i].onclick=function(){
           return !showpic(this);
        }
    }
}
阻止点击链接的默认事件没有成功 哪里错了

没看出来你在哪里写了阻止的代码，你代码里的links的onclick只是添加了功能

百度一下阻止方法，这是window.evevt(IE或谷歌里）或参数event（火狐）的功能。