Already at coverages as low as 0.005 monolayer of potassium on room temperature Si(111)7x7 surfaces, atomic resolution is gradually lost when imaging the surface with scanning tunneling microscope at positive sample bias, giving rise to bright triangles over the faulted halves of the 7x7 unit cells and dark triangles above the unfaulted halves. We suggest that this is due to potassium atoms that are picked up by the tip from K islands at the surface, as is evident by the observed lowering of the tunneling energy barrier. This phenomenon vanishes upon thermal desorption of K islands or upon exposure to 0.1 L oxygen.