Left panel: probability distribution function, obtained from the set of 20 simulations, of the Δ_rms offset between the perturbed and best-fitting model-predicted positions of the 24 multiple images. The predicted positions are determined by optimizing the parameters of the total mass distribution of the main lens not including the mass perturbers along the line-of-sight. Middle and right panels: best-fitting values of the parameters Ω_m, Ω_Λ and w from the set of 20 simulations. These values are determined by optimizing the parameters of the total mass distribution of the main lens and of the cosmological model not including in the mass perturbers along the line-of-sight.