  Testing gravity theory in the strong field region becomes a reality due to
the observations of gravitational waves and black hole shadows. In this paper,
we discuss how to constrain the possible deviations of the classical general
relativity with the image of M87* observed by the Event Horizon Telescope. More
precisely, we want to know where is the event horizon for a non-rotating black
hole. General relativity predicts the horizon is located at the Schwarzschild
radius $r_\textrm{s}$, while other gravity theories may give different
predictions. We propose a parameterized Schwarzschild metric (PSM) in which the
horizon is located at $r=nr_\textrm{s}$, where $n$ is a real free parameter,
and prove general relativity with nonlinear electrodynamics allows $n\neq1$. In
the weak field region, the PSM is equivalent to the Schwarzschild metric
regardless of the value of $n$. In the strong field region, the difference
between the PSM and Schwarzschild metric would leave an imprint on the shadow
image. We present detailed calculations and discussions on the black hole
shadows with large background light source and accretion disk in the PSM
framework. More importantly, we point out that $n\approx2$ can be used to
explain why the black hole mass measured by the shadow is a factor of about two
larger than the previous gas dynamics measurements. If this explanation is
confirmed to be right, then this phenomenon, together with the late-time
cosmological acceleration, will be very important to test gravity theories.
