1. A method of generating a diagnostic image, the method comprising: determining a wavelength band of light maximally absorbed by a photoacoustic material, based on at least one or more photoacoustic materials injected into an object; irradiating light, which corresponds to the determined wavelength band, onto an internal region of interest (ROI) of the object and acquiring a photoacoustic signal corresponding to the irradiated light; generating a photoacoustic image indicating the ROI by using the photoacoustic signal; and combining the generated photoacoustic image and an image expressing that the light is being irradiated onto the ROI and displaying the combined image.