  We determine analytically the quantum Cram\'er-Rao bound for the estimation
of the separation between two point sources in arbitrary Gaussian states. Our
analytical expression is valid for arbitrary sources brightness, and it allows
to determine how different resources, such as mutual coherence (induced by
thermal correlations or displacement) or squeezing affect the scaling of the
ultimate resolution limit with the mean number of emitted photons. In practical
scenarios, we find coherent states of the sources to achieve quantum optimal
resolution.
