TCPDF ERROR: [Image] Unable to get image
This error message can appear when a lot of images are being resized for inclusion into the pdf and the server runs a resource limit like available memory.
Since the resized images are kept, depending on your catalog size and variability, this issue could solve itself over time. One other alternative could be to use the same resized image you are using for your frontend in the pdfs. To do this you need to know the image size that your template uses.
Then edit /app/code/community/Fooman/PdfCustomiser/Model/Mypdf.php and find
$imageHelper = Mage::helper('catalog/image')->init($product, 'image')->resize(500);
Change the resize to the same dimensions as your product media template.