The problem of the low-temperature resistance of granular films of manganites with nanometric grain sizes is addressed. The observed upturn in the low-temperature resistance is understood in terms of charging effects. However, the standard Coulomb blockade scenario does not describe the observed dependence of the charging energies on the applied magnetic field. We propose a theoretical framework in which a distribution of charging energies exists, due mainly to the randomness in the intergranular conductances and not in the grain diameters. The increase of the conductances with the magnetic field induces a renormalization of the charging energies that explain the experimental observations.