1. An image display apparatus, comprising; a CPU configured to perform functions of at least one of an input unit, a determination unit, a margin adding unit, a plurality of image processing units, and a synthesizing unit; the input unit that inputs, from external image output apparatus, a plurality of input images to an edge of each of which a margin image has not been added; the determination unit that determines whether the plurality of input images are a plurality of divided images obtained by dividing one original image, or a plurality of independent images which are respectively independent; the margin adding unit that (i) adds a first margin image to an edge of a first input image of the plurality of input images, and adds a second margin image to an edge of a second input image that is an input image adjacent to the first input image in the original image, in a case where the plurality of input images are determined by the determination unit to be the plurality of divided images, and (ii) does not add the first margin image to the first input image, and does not add the second margin image to the second input image, in a case where the plurality of input images are determined by the determination unit to be the plurality of independent images; the plurality of image processing units that perform an image processing on the plurality of input images; the synthesizing unit that synthesizes the plurality of input images on which the image processing has been performed by the plurality of image processing units; and a display unit that displays the plurality of input images synthesized by the synthesizing unit, wherein in a case where the plurality of input images are determined by the determination unit to be the plurality of divided images, the first margin image is generated from an edge portion of the second input image, the second margin image is generated from an edge portion of the first input image, the image processing includes spatial filter processing in which a pixel value of a target pixel is rewritten to a value generated by weighting and synthesizing the pixel value of the target pixel and pixel values of neighboring pixels with weights according to their pixel positions, (i) in case where the plurality of input images are determined by the determination unit to be the plurality of divided images, a first image processing unit of the plurality of image processing units performs the image processing on the first input image to which the first margin image has been added, and a second image processing unit of the plurality of image processing units performs the image processing on the second input image to which the second margin image has been added, and wherein (ii) in a case where the plurality of input images are determined by the determination unit to be the plurality of independent images, the first image processing unit performs the image processing on the first input image to which the first margin image has not been added, and the second image processing unit performs the image processing on the second input image to which the second margin image has not been added.