1. An image processing device configured to be installed in a vehicle, comprising: an image acquirer configured to acquire camera images captured by cameras provided on the vehicle; an image selector configured to calculate, for each of the camera images, an average value of luminances of respective pixels constituting each of the camera images, and to select one of the camera images having a highest average value as a representative image; a first luminance adjuster configured to adjust a luminance of at least one of the other camera images based on a luminance of the representative image; a synthetic image generator configured to generate a synthetic image showing a periphery of the vehicle, based on the representative image and the other camera images the luminance of at least one of which has been adjusted by the first adjuster; and an image provider configured to output, to a display device installed in the vehicle, information corresponding to the synthetic image.