  The knowledge of the redshift of multiple images in cluster-lenses allows to
determine precisely the total projected mass within the Einstein radius. The
observation of various multiple images in a same cluster is opening new
possibilities to constrain the curvature of the universe. Indeed, although the
influence of $\Omega_m$ and $\Omega_\lambda$ on the images formation is of the
second order, observations of many multiple images at different redshifts
formed by a regular cluster-lens should allow to constrain very accurately the
mass distribution of the cluster and to start to be sensitive to the
cosmological parameters entering the diameter angular distances. We present,
analytical expressions and numerical simulations that allow us to compute the
expected error bars on the cosmological parameters provided an HST/WFPC2
resolution image and spectroscopic redshifts for the multiple images. Numerical
tests on simulated data confirm the rather small uncertainties we could obtain
this way for the two popular cosmological world models: $\Omega_m=0.3{\pm
0.24}$, $\Omega_\lambda=0.7{\pm 0.5}$ or $\Omega_m=1.{\pm 0.33}$,
$\Omega_\lambda=0.{\pm 1.2}$.
