1. An image processing device, comprising: a processor configured to control the operation of the device to form operating units, the operating units comprising: an additional image retrieval unit configured to retrieve an additional image having a first predetermined density, at least a part of the additional image being overlaid with a main image having a second predetermined density, the main image being formed on an image formation medium; a boundary image formation unit configured to form a boundary image which is formed in an area defined in the vicinity of an overlaid area where the additional image is overlaid with the main image, a density of the boundary image being lower than both the first predetermined density and the second predetermined density, the boundary image formation unit including a main image density detection unit configured to detect the second predetermined density, an additional image density detection unit configured to detect the first predetermined density, and a density judgment unit configured to judge whether the first predetermined density is higher than the second predetermined density; an area identifying unit configured to identify an area occupied by the additional image to include: a) an overlaid area where the additional image is overlaid on the main image, b) a boundary image area adjacent to the overlaid area, and c) an additional image area which is defined as an area which is occupied by the additional image with the overlaid area and the boundary image area being excluded therefrom; and a synthesized image formation unit configured to synthesize the main image, the additional image within the additional image area and the boundary image to form a synthesized image; wherein the boundary image formation unit forms the boundary image such that the boundary image is discontinuous in the direction where the main image transverses the additional image if the density judgment unit judges that the first predetermined density is higher than the second predetermined density, while the boundary image formation unit forms the boundary image such that the boundary image is continuous in the direction where the main image transverses the additional image if the density judgment unit judges that the first predetermined density is lower than the second predetermined density.