1. An image output 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 dots according to a result of the preset series of image processing to form an output image on an output medium, said image processing device comprising: a pixel group tone value specification module that specifies a pixel group tone value as a representative tone value of each pixel group, which is provided by collecting a preset number of plural pixels among a number of pixels constituting the image, according to image data of individual pixels in the pixel group; a correlation map storage module that stores a correlation map correlating dot number data of each pixel group, which represents number of dots to be created in the pixel group, to each combination of a pixel group classification number allocated to the pixel group and the specified pixel group tone value of the pixel group; and a dot number data supply module that refers to the stored correlation map to generate dot number data of each pixel group and supplies the generated dot number data of each pixel group to said image output device, said image output device comprising: a pixel position determination module that stores a priority order of dot-on pixels in one pixel group and determines positions of dot-on pixels in each pixel group based on the priority order of dot-on pixels and the supplied dot number data of the pixel group; and a dot creation module that actually creates dots at the determined positions of dot-on pixels on the output medium.