1. An image generation apparatus that generates an image for display on a display installed in a vehicle, the image generation apparatus comprising: an image generator that generates a composite image showing a surrounding area of the vehicle viewed from virtual viewpoints, by combining a plurality of shot images taken by a plurality of on-vehicle cameras that are disposed at different locations on the vehicle; a receiver that receives a signal indicating a lighting mode of a lighting apparatus included in the vehicle; and a controller that changes data volumes of two shot images taken by two of the on-vehicle cameras based on the lighting mode of the lighting apparatus, the data volumes being used in an overlap area of the composite image that corresponds to the surrounding area of the vehicle where the two on-vehicle cameras can shoot in overlap.