1. An image processing device that generates dot data representing dot on-off state in respective pixels of an image and supplies the generated dot data to a printing device, the printing device creating multiple dots on a printing medium to print an image and being designed to print the image with at least a first ratio of a printing resolution in a main scanning direction to a printing resolution in a sub-scanning direction, the image processing device comprising: a halftoning module that applies one specified setting of a halftoning parameter, which corresponds to the first ratio, to a halftoning process to generate dot data from image data representing the image, when the image printed with the first ratio by the printing device according to the dot data generated with the specified setting of the halftoning parameter corresponding to the first ratio has an uneven printing result, the halftoning module applying another specified setting of the halftoning parameter, which corresponds to a second ratio different from the first ratio, to the halftoning process to generate another dot data from the image data.