1. An image synchronization method for three-dimensional (3D) display, comprising: receiving a plurality of first-eye image frames and a plurality of second-eye image frames; selecting a first image frame from the first-eye image frames according to a system time; selecting a second image frame from the second-eye image frames according to a first timestamp of the first image frame; and outputting the first image frame and the second image frame, wherein the step of selecting the first image frame from the first-eye image frames according to the system time comprises: obtaining the system time; obtaining a second timestamp of a first candidate image frame from the first-eye image frames: calculating a first time difference between the system time and the second timestamp; and selecting the first candidate image frame as the first image frame when the first time difference is smaller than a first threshold.