1. An image forming method, comprising: extracting, by a processor, a bitmap image from print data; converting the extracted bitmap image to a sketch image; replacing the bitmap image with the sketch image in the print data; and printing the print data in which the bitmap image is replaced with the sketch image, wherein the converting the extracted bitmap image comprises: enhancing a contrast of the extracted bitmap image; converting the bitmap image with the enhanced contrast to a grayscale image; generating a mask by means of an edge detection of the grayscale image; and multiplying each of color channels of the bitmap image having the enhanced contrast by the mask to generate the sketch image.