In some cases, you may see a "Liquid error:internal" message instead of the product images in your document.
This error message can show up in the documents if the person using the Shopify Order Printer app does not have permissions for "Products" on their staff account.
For some strange reason, the Shopify Order Printer app requires that a user has permission for Products, for the product images to correctly show, and avoid that error message.
Not sure why they did it like that, but adding that permission in the Shopify Admin for the user fixes the issue.
Note: This applies if the error message appears in ALL products listed in the document.