Banners on the side of my forum

[DaVitsche]

We changed the width to fixed to allow banners being put on the side of our forum.

At first it looked like it worked when we put this code at the bottom of the template Navigation / Breadcrumbs templates > navbar

<div style="float:left;
     margin-left:-160px;">
<a href="http://www.website.com"><img src="http://bannerlocation.com/banner.gif"></a>
</div>

<div style="float:right;
     margin-right:-160px;">
<a href="http://www.website.com"><img src="http://bannerlocation.com/banner.gif"></a>
</div>

On the CMS (homepage) it shows ok, but on the forum it stretches the space between the breadcrumbs and the forum with the length of the banner.
Is our approach wrong? should we be putting the code in another template?

EDIT: added a screenshot