<script type="text/javascript">
function now(){
	return (new Date).getTime();
}
var Time = now();
</script>
<img src="http://javascript.ru/files/onready/img5.php" onload="alert(now()-Time)">

<script type="text/javascript">
function now(){
	return (new Date).getTime();
}
var timer = 0,
loadimg = function(img){
timer = now();
img.src = 'http://javascript.ru/files/onready/img5.php';
},
loadedimg = function(img){
var loadtime = now()-timer;
alert(loadtime);
};
</script>
<img src="javascript:true;" onerror="loadimg(this)" onload="loadedimg(this)" alt="Загрзука.." title="Загрзука..">

<script type="text/javascript">
function now(){
    return (new Date).getTime();
}
var firstErr = true, firstLoad = true, time;
function isError(img){
	if(firstErr){
		time = now();
		img.src = 'http://javascript.ru/files/onready/img5.php';
		firstErr = false;
	} 
}
function isLoad(img){
	if(firstLoad&&window.opera) firstLoad = false;
	else alert(now()-time)
}
</script>
<img src="javascript:;" onerror="isError(this)" onload="isLoad(this)"  alt="Загрзука.." title="Загрзука..">

<script type="text/javascript">
function now(){
    return (new Date).getTime();
}
var time;
function isError(img){
	time = now();
	img.src = 'http://javascript.ru/files/onready/img5.php';
}
function isLoad(img){
	alert(now()-time)
}
</script>
<img src="about:blank" onerror="isError(this);" onload="isLoad(this)"  alt="Загрзука.." title="Загрзука..">

<script type="text/javascript">
//Чем раньше ставим этот код, тем лучше
var LoadCounter={};

(function(){
var ERR_TIME = 'pict_load_time';
var LOAD_TIME = 'time';
var id = 'mysite.ru';
var startTime = new Date();
var resultStr = '';
var errorTime;

function now(){
   return (new Date).getTime();
}

function sendData() {
   if (location.host.search(id) == -1)
       return false;
   var img = document.createElement('img');
   var host = location.host.replace(/^www\./, '');
   img.src = 'http://mysite.ru/log_time_download.php?id='+id+'&url='+escape(host)+escape(location.pathname)+resultStr;
   img.style.position = 'absolute';
   img.style.left = '-5px';
   img.style.top = '-5px';
   img.style.width = '1px';
   img.style.height = '1px';
   document.body.insertBefore(img, document.body.firstChild);
}

LoadCounter.isError = function(el) {
   errorTime = now();
   if (el)
       el.src = 'http://mysite.ru/1px.gif?r='+Math.random();
}

LoadCounter.isLoad = function() {
   var secondTime = now() - errorTime;
   if (resultStr) {
       resultStr += '&'+ERR_TIME+'='+secondTime;
       sendData();
   } else {
       resultStr += '&'+ERR_TIME+'='+secondTime;
   }
}

LoadCounter.simpleLoad = function() {
   var simpleTime = now() - startTime;
   if (resultStr) {
       resultStr += '&'+LOAD_TIME+'='+simpleTime;
       sendData();
   } else {
       resultStr += '&'+LOAD_TIME+'='+simpleTime;
   }
}
})()
</script>

где-нибудь в серединке:
<script type="text/javascript">
document.write('<img src="http://mysite.ru/1px.gif?r='+Math.random()+'" onload="LoadCounter.simpleLoad();" />');
</script>
<img src="about:blank" onerror="LoadCounter.isError(this)"  onload="LoadCounter.isLoad(this)" />