If you're talking about your header image, it depends on the contents of your header template. If you've not customized or changed the coding in the template, then look for
<td align="$stylevar[left]"><a href="$vboptions[forumhome].php$session[sessionurl_q]"><img src="$stylevar[titleimage]" border="0" alt="$vboptions" /></a></td>and change the left to center.
If you're talking about a simple banner elsewhere on the site, then you can wrap the code in div tags -
<div align="center">[B]code</div>