1. An image output control system comprising an image processing device that makes image data subjected to a preset series of image processing, and an image output device that creates multiple different types of dots having different densities per unit area according to a result of the preset series of image processing, so as to output an image, said image processing device comprising: a dot number determination module that determines a number of dots to be created in each pixel group, which is set to have a predetermined number of multiple pixels having a two-dimensional size comprising multiple pixels arranged in primary and secondary scan directions included in the image, with respect to each of the multiple different types of dots according to the image data; and a number data output module that outputs the determined number of dots to be created in the pixel group with respect to each type of dot, as dot number data of the pixel group, to said image output device, said image output device comprising: a number data receiving module that receives the dot number data of the pixel group with respect to each type of dot; a priority order specification module that specifies a priority order of individual pixels in the pixel group for dot creation; a pixel position determination module that determines positions of dot-on pixels in the pixel group with respect to each type of dot, based on the dot number data of the pixel group with respect to each type of dot and the specified priority order; and a dot formation module that creates the multiple different types of dots at the determined positions of the dot-on pixels, wherein said dot number determination module comprises: a first dot density data generation module that generates first dot density data representing a density of a first dot to be created in the pixel group, based on the image data, where the first dot has a highest density per unit area among the multiple different types of dots; a second dot density data generation module that generates second dot density data representing a density of either of the first dot and a second dot to be created in the pixel group, based on the image data, where the second dot has a second highest density per unit area among the multiple different types of dots; a threshold value group storage module that stores a threshold value group consisting of multiple threshold values, which respectively correspond to the predetermined number of multiple pixels included in the pixel group; a first dot number determination module that compares the first dot density data with the threshold values included in the threshold value group and sets a number of threshold values that are smaller than the first dot density data to a number of the first dots to be created in the pixel group; and a second dot number determination module that compares the second dot density data with the threshold values included in the threshold value group and sets a number of the second dots to be created in the pixel group, based on the preset number of the first dots and a number of threshold values that are smaller than the second dot density data, said second dot number determination module comparing the second dot density data with only threshold values that are greater than the first dot density data and counting the number of the threshold values that are smaller than the second dot density data, so as to set the number of the second dots to be created in the pixel group.