We demonstrate some unique properties of a rough silver electrode surface revealed by the atomic-resolution electrochemical scanning tunneling microscopy (ECSTM) measurements. Taking the well-known system of iodine adsorption, we observed distinctly different iodine adlayer structures and surface diffusion behavior on mechanically polished polycrystalline silver electrodes in comparison with those on single-crystal electrodes. The results indicate the need to extend atomic-resolution STM studies to rough surfaces of practical importance.