How to bring in scanned images as inserts in invoices or quotes and e-mails?
This is not supported. What are you trying to achieve?
Sometime customer needs a diagram of what I have described in text with dimensions and it is safer to send diagram as an insert to avoid misplacement.
I think this is out of scope. At least for the foreseeable future. I don’t think any general-purpose accounting software is capable of this anyway. Just send your diagrams as separate documents with an invoice.