I was trying to figure out how to do this for a while and wound up taking the div's out of your code in the header template and just wrapping center tags around the rest..
Code:
<center><a name="top" href="{vb:raw vboptions.forumhome}.php{vb:raw session.sessionurl_q}" class="logo-image"><img src="{vb:stylevar titleimage}" alt="{vb:rawphrase x_powered_by_vbulletin, {vb:raw vboptions.bbtitle}}" /></a></center>
Works great and floats when the page size is adjusted.