1. An image processing device comprising: a processor; and a memory storing computer-readable instructions therein, the computer-readable instructions, when executed by the processor, causing the image processing device to perform: acquiring size data indicating a size of a rectangular region, the size data including a first pixel number and a second pixel number, the first pixel number indicating number of pixels aligned in a first direction of the rectangular region, the second pixel number indicating number of pixels aligned in a second direction orthogonal to the first direction of the rectangular region; acquiring first image data representing a first image and second image data representing a second image; generating intermediate image data representing an intermediate image in which the first image is combined with the second image in such a manner that at least a part of the first image and at least a part of the second image are arranged in side-by-side, number of pixels of the intermediate image being larger than each of the number of pixels of the first image and the number of pixels of the second image; and generating rectangular image data representing a rectangular image by executing at least one of addition of at least one pixel to the intermediate image and deletion of at least one pixel from the intermediate image such that number of pixels aligned in the first direction of the rectangular image is brought into coincidence with a third pixel number and number of pixels aligned in the second direction of the rectangular image is brought into coincidence with a fourth pixel number, the third pixel number being obtained by using the first pixel number, the fourth pixel number being obtained by using the second pixel number, a ratio of the third pixel number to the fourth pixel number being equal to a ratio of the first pixel number to the second pixel number.