This recipe shows how to hide the top bar and to put the logo in the left bar.
This line loads the extra style definition:
   * Set USERSTYLEURL=%ATTACHURL%/notopbar.css
You can write this line in 
TWikiPreferences, in 
WebPreferences, in your user topic or on a single page.
Add the dynamic variable link to the logo image to the topic text:
<style type="text/css" media="all">
#patternLeftBar { background-image:url("%PUBURLPATH%/%WEB%/%WEBPREFSTOPIC%/logo.gif"); }
</style>

 If you don't write this overloading style in a template and use an external 
.css file, you need to set the image to the absolute url:
<style type="text/css" media="all">
#patternLeftBar { background-image:url("/twiki/pub/TWiki/WebPreferences/logo.gif"); }
</style>
You can always write a 
<style> in a topic - all current browsers support this - but the page won't validate as valid XHTML.