An image forming apparatus that outputs a plurality of images having different sizes, the image forming apparatus comprising: a communication interface; at least one processor; and an image forming job performer; wherein the communication interface is to obtain from an external apparatus an image forming job comprising a plurality of images; the at least one processor is to: detect respective sizes of the plurality of images in the image forming job, and in response to a determination that at least one of the respective sizes of the plurality of images is not identical to another of the respective sizes of the plurality of images, set the respective sizes of the plurality of images based on a pre-set output mode; and the image forming job performer is to perform an outputting operation based on the respective set sizes of the plurality of images, characterized in that when the output mode is pre-set to an automatic alternative output mode, the at least one processor is to: detect sizes of paper in paper trays of the image forming apparatus, compare the respective sizes of the plurality of images to the sizes of the paper in the paper trays, and when at least one size from among the detected sizes of the plurality of images does not match any size of the paper in the paper trays, set the at least one size to a specific size. a communication interface; at least one processor; and an image forming job performer; wherein the communication interface is to obtain from an external apparatus an image forming job comprising a plurality of images; the at least one processor is to: detect respective sizes of the plurality of images in the image forming job, and in response to a determination that at least one of the respective sizes of the plurality of images is not identical to another of the respective sizes of the plurality of images, set the respective sizes of the plurality of images based on a pre-set output mode; and the image forming job performer is to perform an outputting operation based on the respective set sizes of the plurality of images, characterized in that when the output mode is pre-set to an automatic alternative output mode, the at least one processor is to: detect sizes of paper in paper trays of the image forming apparatus, compare the respective sizes of the plurality of images to the sizes of the paper in the paper trays, and when at least one size from among the detected sizes of the plurality of images does not match any size of the paper in the paper trays, set the at least one size to a specific size. detect respective sizes of the plurality of images in the image forming job, and in response to a determination that at least one of the respective sizes of the plurality of images is not identical to another of the respective sizes of the plurality of images, set the respective sizes of the plurality of images based on a pre-set output mode; and the image forming job performer is to perform an outputting operation based on the respective set sizes of the plurality of images, characterized in that when the output mode is pre-set to an automatic alternative output mode, the at least one processor is to: detect sizes of paper in paper trays of the image forming apparatus, compare the respective sizes of the plurality of images to the sizes of the paper in the paper trays, and when at least one size from among the detected sizes of the plurality of images does not match any size of the paper in the paper trays, set the at least one size to a specific size. detect sizes of paper in paper trays of the image forming apparatus, compare the respective sizes of the plurality of images to the sizes of the paper in the paper trays, and when at least one size from among the detected sizes of the plurality of images does not match any size of the paper in the paper trays, set the at least one size to a specific size.