Electrons trapped in field emission resonances (FERs) in front of a Ag(100) surface covered with ultrathin NaCl islands are probed by scanning tunneling spectroscopy. The eigenstates in the potential well formed between the tip and the sample are calculated within a one-dimensional model. This approach permits to locally determine a work function reduction of 1.3 eV in going from the bare substrate to NaCl islands of up to 3 ML. Spatial mapping of the FERs across a NaCl island edge at typical distances of 1 nm from the surface yields a lateral resolution for the surface potential changes of 1 nm.