1. An image processing method for an image forming apparatus having a nozzle for ejecting an ink droplet onto a printing medium, comprising the steps of: obtaining an original image; storing the original image in a storage medium; converting a gradation of a pixel constituting the original image stored in the storage medium in at least two different manners to generate converted original images be expressed by the image forming apparatus; outputting a dot pattern image through the nozzle of the image forming apparatus based on a predetermined dot pattern signal; generating nozzle characteristic information indicating a characteristic of the nozzle based on the dot pattern image; outputting at least two sets of simulation information based on the converted original images and the nozzle characteristic information; storing the at least two sets of simulation information in a storage medium; generating converted simulation information by converting gradation of a pixel of the at least two sets of the simulation information to the same level as the original image; comparing the at least two sets of the converted simulation information with the original image; calculating a conversion error between the original image and the at least two sets of the converted simulation information; storing the conversion error in a storage medium; and specifying one of the converted original images as drawing information based on the conversion error to cause the image forming apparatus to form the image by ejecting an ink droplet.