Ah got it. Yes, I use a custom template to include images. Sorry, wasn′t aware that thats not standard. I found below code in the template. I guess that this is creating the image in the Pdf file.
<tbody>
<?php foreach ( $line_items as $item_id => $item ) : $_product = $item->get_product(); ?>
<tr class="<?php echo apply_filters( 'wcfm_packing_slip_item_row_class', $item_id, $document_type, $order, $item_id ); ?>">
<td class="product" style="width:30%">
<style>
.product_image { width: 60px; height: 60px; }
.product_image img { width: 100%; height: 100% }
</style>
<?php
if (is_object($_product)) {
echo '<div class="product_image">' . $_product->get_image() . '</div>';
}
?>
<span class="item-name"><?php echo esc_html( apply_filters( 'wcfm_order_item_name', $item->get_name(), $item ) ); ?></span>
<?php
if ( $_product && $_product->get_sku() ) {
echo '<div class="wc-order-item-sku"><strong>' . __( 'SKU:', 'wc-frontend-manager' ) . '</strong> ' . esc_html( $_product->get_sku() ) . '</div>';
}
if ( ! empty( $item->get_variation_id() ) ) {
echo '<div class="wc-order-item-variation"><strong>' . __( 'Variation ID:', 'wc-frontend-manager' ) . '</strong> ';
if ( ! empty( $item->get_variation_id() ) && 'product_variation' === get_post_type( $item->get_variation_id() ) ) {
echo esc_html( $item->get_variation_id() );
} elseif ( ! empty( $item->get_variation_id() ) ) {
echo esc_html( $item->get_variation_id() ) . ' (' . __( 'No longer exists', 'wc-frontend-manager' ) . ')';
}
echo '</div>';
}
?>