1. A method of displaying an image, comprising: receiving an input image; receiving a power decrease ratio from a user; calculating a brightness of an output image corresponding to the input image, which can offset a luminance decrease ratio decreased depending on the power decrease ratio; controlling the size of each component of a pixel constituting the input image in accordance with the calculated brightness of the output image; and displaying the output image of the pixel whose component size has been controlled, based on a light source driven by a power decreased by the power decrease ratio, wherein the calculating the brightness includes: obtaining a difference between a luminance value corresponding an initial power and a luminance value corresponding to the power decreased from the initial power by the power decrease ratio; and calculating a brightness of the output image to make the difference equal to a difference between a luminance value corresponding to the brightness of the input image and a luminance value corresponding to the brightness of the output image.