In the presence of an external magnetic field, the atomic dipoles of paramagnetic substances are aligned along the field. The material gets magnetised and it produces an extra magnetic field in the material in the direction of the field. So for paramagnetic materials, the resultant field is greater than the applied field. As the body of a frog behaves like a paramagnetic substance, the smallest magnetic field on the body of the frog will be strong enough to lift it up.