The general field of the invention is that of the methods for graphically representing a first image from an image sensor of the outside landscape superimposed on a second image representing a synthetic image of the same outside landscape, the two images being displayed on a display screen of an onboard display system for aircraft. The first image comprises three rectangular zones of increasing transparency, each zone having a width equal to that of the first image and a determined height, the sum of the three heights being equal to the height of the first image, the first zone situated at the bottom of the image having a first constant level of transparency, the third zone situated at the top of the image having a second constant level of transparency greater than the first level of transparency, the second zone situated between the first zone and the third zone having a level of transparency that is continuously variable between the first level and the second level.