  In fully general relativity, we calculate the images of the radiatively
inefficient accretion flow (RIAF) surrounding a Kerr black hole with arbitrary
spins, inclination angles, and observational wavelengths. For the same initial
conditions, such as the fixed accretion rate, it is found that the intrinsic
size and radiation intensity of the images become larger, but the images become
more compact in the inner region, while the size of the black hole shadow
decreases with the increase of the black hole spin. With the increase of the
inclination angles, the shapes of the black hole shadows change and become
smaller, even disappear at all due to the obscuration by the thick disks. For
median inclination angles, the radial velocity observed at infinity is larger
because of both the rotation and radial motion of the fluid in the disk, which
results in the luminous part of the images is much brighter. For larger
inclination angles, such as the disk is edge on, the emission becomes dimmer at
longer observational wavelengths (such as at 7.0mm and 3.5mm wavelengths), or
brighter at shorter observational wavelengths (such as at 1.3 mm wavelength)
than that of the face on case, except for the high spin and high inclination
images. These complex behaviors are due to the combination of the Lorentz
boosting effect and the radiative absorption in the disk. We hope our results
are helpful to determine the spin parameter of the black hole in low luminosity
sources, such as the Galactic center. A primary analysis by comparison with the
observed sizes of Sgr A* at millimeters strongly suggests that the disk around
the central black hole at Sgr A* is highly inclined or the central black hole
is rotating fast.
