  We present a strong-lensing analysis of the galaxy cluster MS 1358.4+6245
($z=0.33$), in deep 6-band ACS/HST imaging. In addition to the well-studied
system at $z=4.92$, our modelling method uncovers 19 new multiply-lensed images
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, $d\log \Sigma/d\log r\simeq -0.33 \pm0.05$ ($r<200$ 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 $z_s=4.92$ galaxy is high, about a $\sim100\times$ over
its four images, so that the inferred source size, luminosity and
star-formation rate are about $\sim5\times$ smaller than previous estimates,
corresponding to a dwarf-sized galaxy of radius $\simeq1$ kpc. A detailed image
of the interior morphology of the source is generated with a high effective
resolution of only $\simeq$50 pc, thanks to the high magnification and to the
declining angular diameter distance above $z\sim1.5$ for the standard
cosmology, so that this image apparently represents the best resolved object
known at high redshift.
