Would be a two part process. Since the ‘signing’ of a document derives form checksum hash against the private key allowing the public key to verify content as unchanged, then adding an image with key would invalidate the checksum. Add the image first then generate the key. Depending on document processor/e-mail you may get to usa a batch file and combine the steps as one command. Basic is still two processes – attach/embed image of signature and then generate ‘signature’.