Admin CP -> Skins & Templates -> HTML Templates ->Manage HTML -> Global HTML elements -> Board Header (logo, links, etc)
Find this:
Code:
<!--IBF.BANNER-->
<div id='logostrip'>
<a href='{ibf.script_url}' title='Board Home'><img src='changeme' alt='Powered by Invision Power Board' border="0" /></a>
</div>
The text hightlighted in red can be replaced with your banner url. The text in blue can be replaced with the name of your forum.