Using an ab initio cluster model, we have investigated the electronic information contained in scanning tunneling microscope (STM) images, which were obtained after atom extraction from the Si(111) 7 x 7 surface. Depending on the position of the created vacancy (corner versus center) and on the number of extracted atoms (single atom versus dimer), we obtain different density maps that permit us to propose a rationalization of the brightness of the spots that are observed in the corresponding experimental STM images. It can be understood by a localization of the surface state near the created atomic vacancy.