  The Event Horizon Telescope's image of the M87 black hole provides an
exciting opportunity to study black hole physics. Since a black hole's event
horizon absorbs all electromagnetic waves, it is difficult to actively probe
the horizon's existence. However, with the help of a family of extremely
compact, horizon-less objects, named gravastars", whose external space-times
nearly identical to those of black holes, one can test the absence of event
horizons: absences of additional features that arise due to the existence of
the gravastar, or its surface, can be used as quantitative evidence for black
holes. We apply Gralla et al.'s approach to studying black hole images to study
the images of two types of gravastars: transparent ones and reflective ones. In
both cases, the transmission of rays through gravastars, or their reflections
on their surfaces, lead to more rings in their images. For simple emission
models, where the redshifted emissivity of the disk is peaked at a particular
radius r-peak, the position of a series of rings can be related in a simple
manner to light ray propagation: a ring shows up around impact parameter b
whenever rays incident from infinity at b intersects the disk at r-peak. We
show that additional rings will appear in the images of transparent and
reflective gravastars. In particular, one of the additional rings for the
reflective gravastar is due to the prompt reflection of light on the gravastar
surface and appears to be well separated from the others. This can be a robust
feature, which may be reliably used to constrain the reflectivity of the black
hole's horizon.
