1. An image forming system, comprising: a data conversion unit that thins out first image data configured by arranging dot data in matrix fashion and converts two-line data of the first image data into one-line data of second image data; and an image output unit that outputs an image obtained by reproducing the second image data converted by the data conversion unit, wherein the data conversion unit includes: a determination unit which divides the first image data into a dot block each includes plural pieces of dot data, and determines whether or not an array of dot data included in each dot block corresponds to a preset specific array; and a replacement unit which, when the determination unit determines that the array in a dot block corresponds to the specific array, replaces the data out of the second image data, which is located in a portion corresponding to the dot block, with first replacement data including image forming dot data with which an image is formed, and wherein the specific array includes a first array in which dot data to be thinned out is configured as the image forming dot data, and dot data not to be thinned out is configured as non-image forming dot data with which an image is not formed.