After searching the forums it was obvious that others might be interested and no solution could be found. So, I'll just answer my own question. But first-off let me say that TCPDF is really hard-core stuff! Would be nicer to have something which converts HTML to PDF...
My wish was to move the logo into the header area and make the background black.
Resources:
http://www.tcpdf.org/examples.php
http://www.tcpdf.org/doc/classTCPDF.html
http://api.joomla.org/com-tecnick-tcpdf/TCPDF.html#methodCell
includes/tcpdf.php
templates/{yours}/invoicepdf.tpl
##### Changes to invoicepdf.tpl:
$pdf->SetHeaderMargin(PDF_MARGIN_HEADER);
$pdf->SetHeaderData('logo.png',50);
$pdf->setHeader();
# Logo
//$pdf->Image(ROOTDIR.'/images/logo.jpg',20,25);
//$pdf->Ln();
##### Changes to tcpdf.php:
# Look for: function Header (around line 2531 in WHMCS v.4.4.2)
# insert this line right before the logo is added (4th line in)
$this->Rect(0,0,1370,15,'F','',array(0,0,0));
##### Also had to create the file and directory:
includes/images/logo.png
And now on to the Footer...