FPDF error: Unsupported image type (while using header to define image)

So I have an image string that I insert into a webpage with a header telling the browser it is an image.

So a page would look like this:

<?php
    header('Content-Type: image/png');
    echo $image_data;
?>

http://website.com/randomString.png is used to grab the image (using .htaccess)

When I use the following code, I get the error FPDF error: Unsupported image type

$imagePath = file_get_contents("http://website.com/".$randomString.".png");
$pdf->Image($imagePath,159,185,27);

Does anyone know what I can do to get rid of this error?

Solution

Replace Image with MemImage (and copy class from the listing below or copy the functions by itself), otherwise it wants to read image from file.