1. An image generating apparatus comprising: a document-image acquiring section that acquires a document image, which is a binary image formed based on an electronic document; an information-image acquiring section that acquires an information image, which is a binary image formed based on predetermined information; a determination section that determines the size of image elements formed by an image forming mechanism connected to the image generating apparatus as a characteristic of the image forming mechanism; and a synthesizing section that performs an exclusive OR operation between the document image and the information image to synthesize the document image and the information image, wherein: the information image includes a plurality of image elements having a predetermined size, the document image includes an image portion where an image of the information is present and a background portion where no image is present, the synthesizing section adjusts a size of an image element of the information image synthesized in the image portion of the document image and a size of an image element of the information image synthesized in the background portion based on a magnitude relation between the size of the image element of the information image synthesized in the image portion of the document image and the size of the image element of the information image synthesized in the background portion of the document image, the magnitude relation being based on the size of image elements determined by the determination section, the synthesizing section increases a size of an image element of the information image synthesized in the image portion of the document image and decreases a size of an image element of the information image synthesized in the background portion of the document image where, based on the magnitude relation determined by the determination section, the size of the image element of the information image synthesized in the image portion of the document image is smaller than the size of the image element of the information image synthesized in the background portion of the document image, and the synthesizing section decreases a size of an image element of the information image synthesized in the image portion of the document image and increases a size of an image element of the information image synthesized in the background portion of the document image where, based on the magnitude relation determined by the determination section, the size of the image element of the information image synthesized in the image portion of the document image is larger than the size of the image element of the information image synthesized in the background portion of the document image.