Hallo,
dieses Skript funktioniert toll, aber nur im IE...
function popup(URL) {
day = new Date();
id = day.getTime();
eval("page" + id + " = window.open(URL, '" + id + "',
'toolbar=0,scrollbars=1,location=0,statusbar=1,menubar=1,resizable=1,width=900,height=680');");
}
<a href="javascript:popup('../ordner_' . $_GET['id'] . '_' . $row['id'] . '.jpg')"><img src="../ordner_' . $_GET['id'] . '_' . $row['id'] . '.jpg" width="400" height="300" /></a>