The default banner is the image across the page top below the menu. If you don't want the banner at all go to the Theme tab, choose your template from the grey drop-downs and click Edit. Where you see the following tag:
<!--Include the banner -->
<?php include('banner.inc.php'); ?>
either delete it or comment out the include line so it looks like this:
<?php /*include('banner.inc.php');*/ ?>
The banner need not be an image. It can be any html, but we are going to assume it is an image for these directions.
Setting a Default Banner
To over-ride the default image make a component, name it banner and write in it whatever html you want to see there. For instance if your new banner is to be an image named banner01.jpg and you have uploaded it through the Files tab to the uploads directory you would write in your component:
<img src="<?php get_site_url(); ?>data/uploads/banner01.jpg" alt="text here">
The image doesn't need to be any particular size or proportion.
Setting Banners for Individual Pages
Banners are set for individual pages in the same way, but the component needs to be named banner-slug where slug is the end of the url for that page. For instance to set a special bannner just for the Homepage you would call the component banner-index