When the page loads, you can trigger the jQuery to change the background-image of your HTML document. It depends on which element you set the background-image, to your body or to your div container because ASP.NET allows you to create a similar layout and then load your website's content into the layout.
Anyway, the code to change the background-image in jQuery is as simple as changing the CSS properties of the elements.
$(document).ready(function () {
$('body').load(function () {
$('element').css('background-image', 'url(location-of-image.png)');
});
});
The above code would update background-image upon the body load event. The element should be replaced by the element with its ID that you want to get the new background-image; can be body too.
I didn't do this the ASP.NET MVC way, because that would require registering the scripts or function upon different application based functions and so on. jQuery way is pretty much simple. :)