  The Johannsen black hole (BH) is a generic rotating BH admitting three
constants of motions (energy, angular momentum, and Carter constant) and is
characterized by four deviation parameters besides mass and spin, which could
be a model-independent probe of the no-hair theorem. We systematically study
the dynamics of null particles around Johannsen BH, revealing the effects of
the deviation parameters on the BH shadow as well as the effects of spin. By
using the shadow boundaries of M87* and SgrA*, for the first time, the
deviation parameters of those BHs are constrained. The detailed results depend
on the spin $a$ and inclination angle $ \theta_0$. Assuming $a=0.2$ and
$\theta_0=15^{\circ}$, the deviation parameter $\alpha_{13}$ are constained
within $\sim $ [-3.5, 6] for M87* observation and [-3, 0.5] for SgrA*
observation. We also show the images of a Johannsen BH surrounded by a
Page-Thorne thin accretion disk observed by a remote observer with a
ray-tracing method and discuss the effects of the deviation parameters on
deforming the accretion disk image, which could be tested by observations with
higher sensitivities in the future.
