in a full ipb, u need to edit the html templates, so go in to Skins & Templates . then select html templates. then select the skin of wich banner you want to edit, then exspand Global HTML elements.
Select Board Header (logo, links, etc), aka click edit single on it, then scroll down, to you see the banners <img src=""> tag etc, and edit the url to your own banner.
or you could simply overide the ornal banner image, by saveing over it with your own banner with the same name.