1. An image registration device comprising: a scanner that acquires a document image that is scanned from a medium on which the document image is formed; and at least one processor which executes: a document ID extraction unit that extracts a document ID from the acquired document image and provides the extracted document ID to a security policy server; a stamp image extraction unit that extracts a stamp image associated with the acquired document image; and a stamp registration unit that, in response to a notification from the security policy server that a security policy has been assigned to the extracted document ID and that the extracted stamp image has not been registered in the security policy server, registers the extracted stamp image as being assigned to the security policy.