An explicit exact analytic expression for the energy of the electrostatic interaction between a platelike particle 1 and a spherical particle 2 of radius a(2) immersed in an electrolyte solution of Debye-Huckel parameter h-is derived on the basis of the linearized Poisson-Boltzmann equation. Both particles may have either constant surface potential or constant surface charge density. In the limit of kappa a(2) --> 0, the interaction between a plate with zero surface charge density and a sphere having constant surface charge density becomes identical to the usual image interaction between a point charge and an uncharged plate. The present theory thus leads to a generalization of the usual image interaction of a point charge to cover the case of a charged colloidal particle of finite size.