i'm not sure i understand what you mean by the white (i don t have lots of white in black pearl except for some text), but it all comes down to what you want to do exactly so maybe you can describe it alittle better?
if you would like to make your banner in .png format you could easily replace /styles/black_pearl/imageset/sitelogo_small.png with your own and set the size for it in /styles/black_pearl/imageset/imageset.cfg (the numbers in there are the size)
if you would do it this way you might want to delete the site-name and site-description code lines from the overall_header.html (to make it center properly)
that code would look something like this:
<td align="center" valign="middle"><h1><font color="#AA0000" size="5px"><b><i><u>{SITENAME}</u></i></b></font></h1><span class="gen"><font color="lime"><b><i>{SITE_DESCRIPTION}</i></b></font></span></td>
for your second question ... there's many ways to do this you could even try using the announcement-centre mod wich would give you several extra options (like showing something different to members and guests, modify/update the table in
ACP, choosing on wich pages it shows, etc.)
whatever you choose to do you need to add the code at the bottom of overall_header.html to place it below the menubar
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