Hello there, @Timetaylor.
I'm here to help add watermark to your invoices and other transactions.
When creating invoice, you may need to manage the template and add the watermark from there. Let me show you how.
Once done, do the same thing if you wish to add watermark to your purchase orders and other transactions.
That would do it. Let me know if you have any other questions, I'm always here to help. Have a good one!