An image processing method includes converting multilevel data of an image into a dot pattern using a multilevel error diffusion process. The conversion includes determining a pixel corresponding to an abnormal nozzle based on abnormal nozzle information provided for each of droplet sizes supported by nozzles of an image forming apparatus; preventing jetting of a droplet onto the determined pixel; distributing a quantization error of the determined pixel calculated in the multilevel error diffusion process to neighboring pixels; and if extra-large droplets with a droplet size greater than the droplet size of a full-size droplet capable of filling a pixel are to be formed in the neighboring pixels as a result of distributing the quantization error, removing one or more of the extra-large droplets or reducing the droplet size of one or more of the extra-large droplets.