The cause is usually the file name: if your logo's file name contains spaces, Shopware shows a placeholder image with a cross or no image at all on invoices, delivery notes, and other documents instead of the logo. A different file format won't help, the name is what matters.
How to fix it
Rename the image file so that its name contains no spaces, for example firmenlogo.png instead of Firmen Logo.png. Then upload it again and select it as the logo again in the document settings of each affected document.
Note: The error is in Shopware and has been fixed as of Shopware 6.7.11.0. If you are working with an older version, renaming the file will help.
To find out how else to configure your documents, see How do I configure and customize my order documents?