Spreadsheet Excel Writer insertBitmap() not working

Hi,

I'm not sure why this isn't working. I've searched around and I've not been able to find anything similar. The image is in the same directory as the spreadsheet file (sprmaker.php). Do I have to make it './image.bmp' maybe? Here's my code:

sprmaker.php:
<?php
require_once 'Spreadsheet/Excel/Writer.php';
$workbook = new Spreadsheet_Excel_Writer();

//create the loop
$num = count($orders)-1;
for($i=0; $i<=$num; $i++)
{
//insert the image
$worksheet[$i]->insertBitmap(0,0,'image.bmp');
}

//send the file
$workbook->send('test.xls');
//close the workbook
$workbook->close();

?>