1. An image processing apparatus, comprising an image size adjustment section, which, in order to synthesize a plurality of image data items to create one image, divides, respectively, the number of pixels, in a vertical direction and in a horizontal direction, of input image data by the maximum value of the ratio of resolutions of the plurality of image data items configuring the synthetic image, and adjusts the image size, wherein the image size adjustment section comprises: remainder computation means for obtaining the ratio from the resolutions of the plurality of image data items, and computing a remainder by dividing, respectively, the number of pixels in the vertical direction and in the horizontal direction of the input image data by the maximum value of the ratio; and size adjusting means for adjusting, respectively, the number of pixels in the vertical direction and in the horizontal direction of the input image data, on the basis of the remainder computed by the remainder computation means, so that the remainder becomes 0.