1. An image synthesis system for a vehicle comprising: a first camera disposed on the vehicle and shooting a first image of the periphery of said vehicle; a second camera disposed on the vehicle and shooting a second image of the periphery of said vehicle, wherein the second image and the first image have an overlap region; an image processing device comprising: a defining component for defining the first image into a plurality of first points each has a first residual error, and defining the second image into a plurality of second points each has a second residual error; and a synthesis component for synthesizing the first image and the second image and output a third image, wherein when the first point overlaps to the second point only the point which has lower residual error will be shown in the third image; and a display device for displaying the third image.