A low-temperature scanning force microscope sensitive to electrostatics has been constructed for investigating a two-dimensional electron system (2DES) under quantum Hall conditions. In order to cope with the highly resistive properties of the 2DES under these conditions, a low-frequency measurement technique is presented, based on the shift of the cantilever's resonance frequency induced by a small ac current modulation within the 2DES. Since the 2DES is buried in a GaAs/AlGaAs heterostructure, a special calibration technique has to be applied, which allows to map Hall-potential profiles with clearly submicron resolution