Later I found a similar problem in tpl_main_page.php but no change was seen in image for about_us page. Home page remained unchanged. I changed (about line 92) 'topmenubg_login.jpg' to gif and 'topmenubg.jpg' to gif.

Corrected login/logout jpg to gif to match image files.
PHP Code:
    <!-- begin top navigation -->    

<?php

     if (STORE_STATUS == '0' and !$_SESSION['customer_id']) { 

            $menubg = 'topmenubg_login.gif';

     } else {

            $menubg = 'topmenubg.gif';

     } 

?>