Hmm the new window will probably be blocked by popup blockers. Maybe this will work for you. Instead of using onclick you can use onmouseover though if you are beginner you probably won't know how to edit the JS. http://www.huddletogether.com/projects/lightbox2/
If you do use that script to change from onclick to onmouseover you would change this line
var Lightbox = Class.create();
Lightbox.prototype = {
// initialize()
// Constructor runs on completion of the DOM loading. Loops through anchor tags looking for
// 'lightbox' references and applies onclick events to appropriate links. The 2nd section of
// the function inserts html at the bottom of the page which is used to display the shadow
// overlay and the image container.
//
initialize: function() {
if (!document.getElementsByTagName){ return; }
var anchors = document.getElementsByTagName('a');
// loop through all anchor tags
for (var i=0; i<anchors.length; i++){
var anchor = anchors[i];
var relAttribute = String(anchor.getAttribute('rel'));
// use the string.match() method to catch 'lightbox' references in the rel attribute
if (anchor.getAttribute('href') && (relAttribute.toLowerCase().match('lightbox'))){
anchor.onclick = function () {myLightbox.start(this); return false;}
}
}