The problem is with your scripts conflicting.
You should organise your scripts like:
<title></title>
<link href="Styles/MainMaster.css" rel="stylesheet" type="text/css" />
<script type='text/javascript' src='/js/jquery.js'></script>
<script type='text/javascript' src='/js/jquery.simplemodal.js'></script>
<script type='text/javascript' src="http://cdn.jquerytools.org/1.2.7/full/jquery.tools.min.js"></script>
<script type="text/javascript">
$(function () {
$("img[title]").tooltip();
});
</script>
<link href="Styles/PopUpWindow.css" rel="stylesheet" type="text/css" />
<asp:ContentPlaceHolder ID="head" runat="server">
</asp:ContentPlaceHolder>