We report on effects from the geometrical shape of the guiding channels on the ion transmission profile. We find that capillaries of rhombic cross section produce rectangular shaped ion transmission profiles and, vice versa, capillaries of rectangular geometry give a rhombic beam shape. Our trajectory simulations for the incidence of 14-keV Ne7+ ions give clear evidence that the observed effect is due to the image forces experienced by the transmitting ions. They gain transverse energy due the image charge attraction towards the inner surfaces of the capillary. This leads to a defocusing of the ions leaving the capillaries. Due to the blocking of large deflection angles at the exit of the capillary, the transmitted ion beam is tailored into certain geometrical patterns.