These print on every invoice and are locked so they cannot be changed by accident. Unlock any one to edit it for this invoice.
These go on every invoice the moment it is filed. Anyone who can sign in can sign; only an admin can replace the images. Replacing one does not touch invoices already filed - they keep the signature that actually signed them.
A PNG with a transparent background, under 300KB. Anything else is refused rather than printed badly.
Every invoice is stored exactly as it printed. Open an old one and it comes back the way it went out - even if the boxes, prices or the footer have changed since.