Problem attaching image to invoice

[David Goodenough]

I am trying attach a scanned PDF to an invoice that I have
entered.  When I entered the invoice and right clicked on any
part of it the the context menu did not contain any mention
of being able to attach a file.

When I posted the invoice and then opened it, then I could
find a context menu entry to attach the file, but when I 
try to attach it, I get an error saying that I am not allowed
to modify a posted invoice.

What is the proper procedure for attaching files to invoice?

I am using version 2.6.11 on a Debian sid system.

Thanks in advance.

David