Ferromagnetic CoFeSiB tips of low coercivity were used in a scanning tunneling microscope to achieve high-resolution magnetic imaging. Magnetic sensitivity is obtained on the basis of local tunneling magnetoresistance between the tip and the sample while the magnetization of the tip is switched periodically. For this method it is crucial to ensure a single-domain configuration at the end of the tip and to avoid mechanical vibrations of the tip due to magnetostriction. We present micromagnetic calculations that show that at the end of the tip only two magnetic states are possible and relate these theoretical expectations with the experiment. Further we demonstrate, that with the proper choice of materials, the magnetostriction can be suppressed well below the noise limit of the experiment and present domain wall profiles of Co(0 0 0 1) with a lateral resolution of at least 1 nm.