1. A method for enlarging or reducing an image on a display screen of a computer system, said method comprising: detecting, by a processor of the computer system, a pointing operation that points to, with a single touch gesture of a pointing means on the display screen, a first figure indicative of a range of the image to be enlarged or reduced and a second figure indicative of a size of the enlarged or reduced range; detecting, on the display screen by the processor, a first trajectory traced by a first movement of a position pointed to on the display screen with a single-touch gesture of a pointing means; detecting, by the processor, a second trajectory traced by a second movement of a position pointed to on the display screen with a single-touch gesture of the pointing means; determining, by the processor, that the second movement tracing the second trajectory occurred after the first trajectory was traced; recognizing the first figure and the second figure; displaying, on the display screen in response to the detection of the pointing operation, an enlarged or reduced image obtained by enlarging or reducing the image at an enlargement or reduction ratio determined based on the first figure and the second figure.