We use a high-vacuum low-temperature scanning tunneling microscope to study the Abrikosov vortex lattice in NbSe2 at 2.7 K. Vortex lattices imaged with a ferromagnetic Ni tip in an external field show a higher vortex density than lattices imaged with a nonmagnetic tip in the same field. The additional field induced by the Ni tip can be extracted from the vortex density and is found to be about 10 mT, independent of the external field. In zero external field, the Ni tip generates vortices with a density corresponding to 10 mT. The scanning motion of the Ni tip neither affects single vortices, nor the complete vortex lattice. We attribute this to the homogeneity of the tip stray field.