1. An image forming apparatus comprising: a scanning unit configured to read a first image, generate first image data representing the first image, read a second image, and generate second image data representing the second image; a processor; memory storing instructions that, when executed, cause the image forming apparatus to: generate synthesized image data representing a synthesized image of the first and second images arranged in one of: a contact state where a first side which is one of four sides constituting an outer edge of the first image and a second side which is one of four sides constituting an outer edge of the second image are disposed immediately adjacent to each other, and a non-contact state in which the first and second sides are disposed parallel to and adjacent to each other with a space therebetween; compare a size in a first axial direction of the synthesized image with the size of a printing area in the first axial direction of an effective printing area of a recording medium; and in response to determining that the size of the synthesized image in the first axial direction is larger than the size of the printing area in the first axial direction, change an arrangement between the first and second images to generate modified synthesized image data representing a modified synthesized image in which another one of the four sides of the first image perpendicular to the first side and another one of the four sides of the second image perpendicular to the second side are disposed in one of: a contact state where the other one of the four sides of the first image and the other one of the four sides of the second image are disposed immediately adjacent to each other, and a non-contact state in which the other one of the four sides of the first image and the other one of the four sides of the second image are disposed parallel to and adjacent to each other with a space therebetween; and a printing unit configured to print, on the recording medium, one of the synthesized image and the modified synthesized image.