We consider the effect of image forces, arising due to a difference in dielectric permeabilities of the well layer and barrier layers, on the energy spectrum of an electron confined in a rectangular potential well under a magnetic field. Depending on the value and the sign of the dielectric mismatch, image forces can localize electrons near the interfaces of the well or in the well centre and change the direct intersubband gaps into indirect ones. These effects can be controlled by variation of the magnetic field, offering possibilities for exact tuning of electronic devices.