An image-processing device and method for enhancing the luminance and the image quality of display panels, the device and method includes a color distribution calculating unit which classifies the original image-color data, and then calculates the ratio of the color data in block B2 to all input image-color data. A control-variable generating unit determines the value of the converting-control variable and the value of the backlight luminance-control variable according to the ratio. The converting-control variable will be output to a numerical converting unit so as to convert the original image-color (RGB) data to the new image-color (R′G′B′W′) data. The backlight luminance-control variable will be output to a backlight luminance-control unit so as to control the backlight luminance.