1. An image processing apparatus, comprising: a document image generator for performing the tasks comprising: generating a verification document image by adding a specific verification pattern including a plurality of verification pattern elements to a background of an original document image, by dividing a selected portion of the original document image into a plurality of equally-sized unit blocks and adding a plurality of verification pattern elements to each one of the plurality of unit blocks only to portions of the unit blocks having no document data thereon, wherein each of the plurality of predetermined verification pattern elements added to each one of the plurality of unit blocks is a unit dot pattern having a plurality of dots; and assigning each one of the plurality of unit blocks a value, and adjusting a number of the plurality of dots of the unit dot pattern added to each unit block using the assigned value to be an adjusted number, wherein the number of the plurality of dot patterns of the unit dot pattern added to each unit block is changed to be the adjusted number based on the assigned value of the respective unit block by deleting a single dot from or adding a single dot to the plurality of dot patterns; and an image processor connected to the document image generator via a network.