We have imaged vortices in the conventional superconductors NbSe2 (crystal) and Nb (thin film) with a low-temperature magnetic force microscope (MFM). The MFM detection is based on commercially available piezoresistive cantilevers. The improved sensitivity (0.2 pN/nm) at 4.3 K has been obtained by using a higher flexural mode of the cantilever. The operation at higher mechanical resonances allows to improve the signal-to-noise ratio by a factor of 3, while limiting the heat dissipation to 50 mu W. The magnetic tip coating was optimized by relying on Co/Au multilayers grown by molecular beam epitaxy. The possibility to image the Abrikosov vortex lattice in NbSe2 can be understood in terms of collective pinning effects. (C) 2000 Elsevier Science B.V. All rights reserved.