Thickbox

You can trigger Lightbox (Thickbox, PrettyPhoto) on page load with the following code.

Fancybox

$(document).ready(function(){
 $("#lightbox").fancybox({
 maxWidth    : 700,
 maxHeight    : 548,
 closeClick    : false,
 }).trigger('click');
});

<a href="http://www.stcloudrox.com/lightbox.html" id="lightbox" class="fancybox fancybox.iframe"></a>

Fancybox w/Cookie (https://github.com/carhartl/jquery-cookie)

function openFancybox() {
 setTimeout( function() {$('#lightbox').trigger('click'); },100);
}
jQuery(document).ready(function() {
 var date = new Date();
 var minutes = 30;
 date.setTime(date.getTime() + (minutes * 60 * 1000));
 var visited = $.cookie('visited');
 if (visited == 'yes') {
 return false;
 } else {
 openFancybox();
 }
 $.cookie('visited', 'yes', { expires: date });
 $('#lightbox').fancybox({
 maxWidth    : 700,
 maxHeight    : 548,
 closeClick    : false
 });
});

PrettyPhoto

$(document).ready(function() {
$.prettyPhoto.open("image","title","description");
});

Thickbox

$(document).ready(function() {
tb_show("", "announcement.html?height=400&width=500");
});