The session_start
function must be called before any html is sent. Best place is on very top of the script:
<?php
session_start();
?>
...
margin-left: 94px;
margin-top: -66px;
margin-left: 94px;
margin-top: -66px;
max-width: 100%;"/></a></h1>
...
See the PHP manual entry for the session_start function, especially the Notes part.