1. An image forming method for use in an inkjet device for forming an image on a printing medium, the image forming method comprising the steps of: providing data representative of the image; providing m data masks for masking the data representative of the image so as to obtain m pieces of image data representing m sub-images, wherein m is an integer greater than one; and printing the m sub-images successively according to the m pieces of image data representing the m sub-images so that the m sub-images are superimposed on the printing medium, whereby the image is formed on the printing medium.