Julhu

I had the same problem as OP. There seems to be an issue when handling images from an external source.

What I did to fix it was adding a filter to convert alle images to base64. With the downside that it will enlarge your PDF’s, but it’s better than not having working thumbnails I guess.

Here is the code I use:

// PDF Invoices & Packing Slips for WooCommerce convert images to base64 as a workaround for not loading images from external sources
add_filter('wpo_wcpdf_get_html', function ($html, $document) {
    // Convert images to base64
    $html = preg_replace_callback('/<img[^>]+>/i', function ($matches) {
        $img = $matches[0];
        $doc = new DOMDocument();
        $doc->loadHTML($img);
        $imageTags = $doc->getElementsByTagName('img');
        foreach ($imageTags as $tag) {
            $src = $tag->getAttribute('src');
            $data = file_get_contents($src);
            $type = pathinfo($src, PATHINFO_EXTENSION);
            $base64 = 'data:image/' . $type . ';base64,' . base64_encode($data);
            $img = str_replace($src, $base64, $img);
        }
        return $img;
    }, $html);
    
    return $html;
}, 10, 2);