1. A computer readable medium having a program including a set of instructions for permitting a computer to perform image processing and display an image on a display screen, said set of instructions comprising: executing a first image generating process which generates an image requiring a small calculation amount for at least one region provided in the display screen; executing a second image generating process which is different from said first image generating process and generates an image requiring a large calculation amount for said at least one region; and displaying sequentially each of the images generated by executing the first image generating process and the second image generating process on the display screen.