The effects of image charges (i.e., induced surface charges of polarization) in spherical geometry and their implication for charged colloidal systems are investigated. We study analytically and exactly a single microion interacting with a dielectric sphere and discuss the similarities and discrepancies with the case of a planar interface. By means of extensive Monte Carlo simulations, we study within the framework of the primitive model the effects of image charges on the structure of the electrical double layer. Salt-free environment as well as salty solutions are considered. A remarkable finding of this study is that the position of the maximum in the counterion density (appearing at moderately surface charge density) remains quasi-identical, regardless of the counterion valence and the salt content, to that obtained within the single-counterion system.