Scanning tunneling microscope images of xenon atoms lying at various sites of a nickel surface are calculated using a Laue representation of the electron scattering states. The simulation program views the tip and the adsorbate as discrete atomic clusters attached to the planar surface of facing Sommerfeld metallic half spaces. It is shown that the elastic electron scattering provides an adequate description of the Xe images : rows of Xe atoms are resolved with a shape in good agreement with experiment. The same model is applied to the study of the recently described one-atom switch of Eigler et al. [Nature 352, 600 (1991)]. The computed current contrast, which adequately compares to observed values, shows significant variations for different Xe adsorption sites on the tip.