You can do this by editing your templates/header.tpl template.
There you have {LOGO} tag on line 38 (inside . top_banner div).
You can remove that tag and place your logo and banner, for example if you upload your logo and banner in templates/img folder, the code would look something like this:
<div class="top_banner">
<img src="templates/img/header.gif" border="0" alt="logo" /> <img src="templates/img/banner.gif" border="0" alt="banner" />
</div>