Do this ... look for the following in your navbar template
Code:
<vb:if condition="$stylevar['titleimage']"><div><a name="top" href="{vb:link forumhome}" class="logo-image"><img src="{vb:stylevar titleimage}" alt="{vb:rawphrase x_powered_by_vbulletin, {vb:raw vboptions.bbtitle}}" /></a></div></vb:if>
and replace it with the following:
Code:
<vb:if condition="$stylevar['titleimage']"><div><a name="top" href="{vb:link forumhome}" class="logo-image"><img src="{vb:stylevar titleimage}" width="315" height="125" title="YOUR TITTLE HERE" alt="AN ALTERNATE DESCRIPTION HERE IF YOU WANT" /></a></div><vb:else /><div> </div></vb:if>
The width and height change it to whatever your header is ...this is one alternative ...
The other is to change the GLOBAL phrase x_powered_by_vbulletin to what you want .... hope this helps ...