confermo che funziona con IE7, Firefox2 e Opera9
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
<head>
    <title>Lightbox JS v2.0 | Test Page</title>
    <link rel="stylesheet" href="css/lightbox.css" type="text/css" media="screen" />

    <script src="js/prototype.js" type="text/javascript"></script>

    <script src="js/scriptaculous.js?load=effects" type="text/javascript"></script>

    <script src="js/lightbox.js" type="text/javascript"></script>

    <style type="text/css">
body{ color: #333; font: 13px 'Lucida Grande', Verdana, sans-serif; }
</style>
</head>
<body>
    <h1>
        Lightbox JS v2.0
    </h1>
    


        by Lokesh Dhakar
    </p>
    <h2>Example</h2>
    <a id="a1" href="images/image-1.jpg" rel="lightbox">
        [img]images/thumb-1.jpg[/img]
    </a>
</body>
</html>

<script type="text/javascript">
//-------------------------------------------------
//Aggiunge azioni all'evento Load
//
//Si può pure fare:
// addEvent(window, "load", func1, false);
// addEvent(window, "load", func2, false);
//-------------------------------------------------

function addLoadEvent(func) 
{
	var oldonload = window.onload;
	if (typeof window.onload != 'function') 
	{
		window.onload = func;
	}
	else 
	{
		window.onload = function() 
		{
			oldonload();
			func();
		}
	}
}
    
addLoadEvent(function(){ window.setTimeout(function(){$("a1").onclick();},1)});    
</script>

confermo pure che non hai dimenticato nulla, tranne naturalmente l'ID dell'immagine da visualizzare

addLoadEvent(function(){ window.setTimeout(function(){$("a1").onclick();},1 )});

L'immagine ha id a1
perciò

<a id="a1" href="images/image-1.jpg" rel="lightbox">****

Buon Natale