We present a strong-lensing analysis of the galaxy cluster MS 1358.4+6245 (z= 0.33), in deep six-band HST/ACS imaging. In addition to the well-studied system at z= 4.92, our modelling method uncovers 19 new multiply lensed images belonging to seven background sources, so that a total of 23 images and their redshifts are used to accurately constrain the inner mass distribution. We derive a relatively shallow inner mass profile, d log Σ/d log r≃−0.33 ± 0.05 (r < 200 kpc), with a much higher magnification than estimated previously by models constrained only by the z= 4.92 system. Using these many new images we can apply a non-parametric adaptive-grid method, which also yields a shallow mass profile without prior assumptions, strengthening our conclusions. The total magnification of the zs= 4.92 galaxy is high, about a ∼100 times over its four images, so that the inferred source size, luminosity and star formation rate are about ∼5 times smaller than previous estimates, corresponding to a dwarf-sized galaxy of radius ≃1 kpc. A detailed image of the interior morphology of the source is generated with a high effective resolution of only ≃50 pc, thanks to the high magnification and to the declining angular diameter distance above z∼ 1.5 for the standard cosmology, so that this image apparently represents the best resolved object known at high redshift.