1. A printing device for discharging inks from nozzles according to a set mode and printing an image on a medium, the printing device comprising: a first setting unit configured to set a printing mode of a main image and to set either a front-surface printing mode for printing a background image on the medium and then printing the main image on the background image or a rear-surface printing mode for printing the main image on the medium and then printing the background image on the main image; and a second setting unit configured to set a printing mode of an additional image printed in association with the printing of the main image and to set either a front-surface printing mode for printing a background image for the additional image on the medium and then printing the additional image on the background image for the additional image or a rear-surface printing mode for printing the additional image on the medium and then printing the background image for the additional image on the additional image, wherein, if the additional image is read by a sensor, the second setting unit sets the printing mode of the additional image to a printing mode for printing an image in which the background image for the additional image is not interposed between the sensor and the additional image.