  We determine the bound to the maximum achievable sensitivity in the
estimation of a scalar parameter from the information contained in an optical
image in the presence of quantum noise. This limit, based on the Cramer-Rao
bound, is valid for any image processing protocol. It is calculated both in the
case of a shot noise limited image and of a non-classical illumination. We also
give practical experimental implementations allowing us to reach this absolute
limit.
