Find the CSS file cubecart.style04.css and add the following:
.page-header-sec {
background-image: url('xyz');
}
where 'xyz' is the location of the image.
In the template main.php:
Find:
<body>
Change to:
<body background="https://mybikerleather.com/Banner.jpg">
or
<body style="background-image:url('https://mybikerleather.com/Banner.jpg');background-repeat:no-repeat;">
Do the same for the template main.checkout.php.