A new image-charge method to calculate the electrostatic potential for a charged particle located between two metal electrodes is presented. In distinction to the conventional methods, which need some procedure of proper choice of parameters, depending on the geometry of the cathode, this one may be used for nonsmooth and complex geometry emitters. The basic idea is to replace one of the electrodes by an image charge surface. The charge distribution along this surface can be calculated with a rather simple and formal procedure. It is shown that the method describes adequately the interaction between a charged particle and electrodes.