1. A display device for displaying a two-dimensional (2D) or a three-dimensional (3D) image, the display device comprising: an image display determination unit which determines whether an input image sequence having a first frame rate is a 2D image sequence or a 3D image sequence; a 2D frame rate conversion unit which receives the input image sequence and generates a 2D output image sequence having a second frame rate if the image display determination unit determines that the input image sequence is the 2D image sequence, wherein the 2D output image sequence comprises the input image sequence and a 2D intermediate image generated from the input image sequence; and a 3D frame rate conversion unit which receives the input image sequence, and generates a 3D output image sequence having a third frame rate if the image display determination unit determines that the input image sequence is the 3D image sequence, wherein the 3D output image sequence repeatedly comprises at least one left-viewpoint image and at least one right-viewpoint image of the input image sequence, a left-viewpoint intermediate image, and a right-viewpoint intermediate image in consideration of an order of the at least one left-viewpoint image and the at least one right-viewpoint image, the left-viewpoint intermediate image is determined from the at least one left-viewpoint image in a left-viewpoint image sequence included in the input image sequence, and the right-viewpoint intermediate image is determined from the at least one right-viewpoint image in a right-viewpoint image sequence included in the input image sequence, wherein a frame rate of the 3D) output image sequence is further increased by inserting a black image between every two adjacent images in the output mages sequence, and wherein at least one of the image display determination unit, the 2D frame rate conversion unit, and the 3D frame rate conversion unit is implemented as a hardware component.