1. An image processing system comprising: a display displaying a first image and displaying a second image, alternately, wherein the first image includes a mark; a camera capturing frames of the first image and capturing frames of the second image; and an image processing device calculating a first difference image between the first image and the second image, detecting a non-zero pixel region of the first difference image, detecting a boundary region of an object in the non-zero pixel region from a second difference image between two of the frames that are captured, removing the boundary region from the non-zero pixel region, and detecting the mark by searching for the mark in the non-zero pixel region of the first difference image from which the boundary region has been removed, wherein the second difference image between the two of the frames that are captured and from which the boundary region of the object is detected are one of: (i) a previous frame of the first image and a current frame of the first image; and (ii) a previous frame of the second image and a current frame of the second image.