watermark with library tcpdf

I need to add a watermark to a pdf generated with tcpdf someone knows how to do it or has some basic example, help please,

I have this example but I need that after the text I put an image as a watermark

require_once('tcpdf/config/lang/eng.php');
require_once('tcpdf/tcpdf.php');


$pdf = new TCPDF('P', PDF_UNIT, 'letter', true, 'UTF-8', false);

$pdf->SetCreator(PDF_CREATOR);
$pdf->SetAuthor('A.C.G.');
$pdf->SetTitle(' EJEMPLO IMG GOTA DE AGUA ');

$pdf->SetHeaderData(PDF_HEADER_LOGO, PDF_HEADER_LOGO_WIDTH, PDF_HEADER_TITLE.' 007', PDF_HEADER_STRING);

$pdf->setHeaderFont(Array(PDF_FONT_NAME_MAIN, '', PDF_FONT_SIZE_MAIN));
$pdf->setFooterFont(Array(PDF_FONT_NAME_DATA, '', PDF_FONT_SIZE_DATA));

$pdf->SetDefaultMonospacedFont(PDF_FONT_MONOSPACED);

//set margins
$pdf->SetMargins(PDF_MARGIN_LEFT, PDF_MARGIN_TOP, PDF_MARGIN_RIGHT);
$pdf->SetHeaderMargin(PDF_MARGIN_HEADER);
$pdf->SetFooterMargin(PDF_MARGIN_FOOTER);

$pdf->SetHeaderData("logo.png", 50, "EJEMPLO IMG GOTA DE AGUA   \n $fechaHora");

$pdf->setHeaderFont(Array('helveticaB', '', PDF_FONT_SIZE_MAIN)); 
$pdf->setFooterFont(Array('helveticaB', '', PDF_FONT_SIZE_DATA)); 

$pdf->SetDefaultMonospacedFont(PDF_FONT_MONOSPACED); 

$pdf->SetMargins(PDF_MARGIN_LEFT, PDF_MARGIN_TOP, PDF_MARGIN_RIGHT); 
$pdf->SetHeaderMargin(PDF_MARGIN_HEADER); 
$pdf->SetFooterMargin(PDF_MARGIN_FOOTER); 


$pdf->SetAutoPageBreak(TRUE, PDF_MARGIN_BOTTOM);

$pdf->setImageScale(PDF_IMAGE_SCALE_RATIO);

$pdf->setLanguageArray($l);


$pdf->SetFont('times', 'B', 12);

$pdf->AddPage();


$html =<<<EOD
<h1>Welcome to <a href="http://www.tcpdf.org" style="text-decoration:none;background-color:#CC0000;color:black;">&nbsp;<span style="color:black;">TC</span><span style="color:white;">PDF</span>&nbsp;</a>!</h1>
<i>This is the first example of TCPDF library.</i>

<p>This text is printed using the <i>writeHTMLCell()</i> method but you can also use: <i>Multicell(), writeHTML(), Write(), Cell() and Text()</i>.</p>
<p>Please check the source code documentation and other examples for further information.</p>
<p style="color:#CC0000;">TO IMPROVE AND EXPAND TCPDF I NEED YOUR SUPPORT, PLEASE <a href="http://sourceforge.net/donate/index.php?group_id=128076">MAKE A DONATION!</a></p>


<h1>Welcome to <a href="http://www.tcpdf.org" style="text-decoration:none;background-color:#CC0000;color:black;">&nbsp;<span style="color:black;">TC</span><span style="color:white;">PDF</span>&nbsp;</a>!</h1>
<i>This is the first example of TCPDF library.</i>
<p>This text is printed using the <i>writeHTMLCell()</i> method but you can also use: <i>Multicell(), writeHTML(), Write(), Cell() and Text()</i>.</p>
<p>Please check the source code documentation and other examples for further information.</p>
<p style="color:#CC0000;">TO IMPROVE AND EXPAND TCPDF I NEED YOUR SUPPORT, PLEASE <a href="http://sourceforge.net/donate/index.php?group_id=128076">MAKE A DONATION!</a></p>


<h1>Welcome to <a href="http://www.tcpdf.org" style="text-decoration:none;background-color:#CC0000;color:black;">&nbsp;<span style="color:black;">TC</span><span style="color:white;">PDF</span>&nbsp;</a>!</h1>
<i>This is the first example of TCPDF library.</i>
<p>This text is printed using the <i>writeHTMLCell()</i> method but you can also use: <i>Multicell(), writeHTML(), Write(), Cell() and Text()</i>.</p>
<p>Please check the source code documentation and other examples for further information.</p>
<p style="color:#CC0000;">TO IMPROVE AND EXPAND TCPDF I NEED YOUR SUPPORT, PLEASE <a href="http://sourceforge.net/donate/index.php?group_id=128076">MAKE A DONATION!</a></p>

EOD;

$pdf->writeHTMLCell(0, 0, '', '', $html, 0, 1, 0, true, '', true);

$pdf->Output('example_001.pdf', 'I');