A device and a method for displaying an image, a device and a method for supplying power, and a method for adjusting brightness of contents are provided. The device for displaying the image includes: a pixel value converter which, if a plurality of color pixel values of the image is received, converts the received color pixel values; a display panel which includes a plurality of color light-emitting devices and which drives each of the plurality of color light-emitting devices based on the converted color pixel values; a light-emission controller which provides the display panel with a control signal which variably controls respective driving times of each of the color light-emitting devices based on colors; and a global controller which controls the light-emission controller to variably adjust a duty ratio of the control signal based on colors and the converted color pixel values.