Sudden changes in the detailed appearance and corrugation of atomic-resolution scanning tunneling microscopy (STM) images are commonly observed. In particular, these effects for the Pt(111)(square-root 3 X square-root 3)R30-degrees-sulfur structure have been studied. Corrugation changes have been investigated by the controlled transfer of atoms between tip and sample. Theoretical image simulations using electron scattering quantum chemical methods show that the, image contrast is strongly dependent on the chemical identity of the STM tip atom. Observed changes in the sulfur images can be explained by changes from platinum-terminated to sulfur-terminated STM tips.