  Direct observation of black holes is one of the grand challenges in
astronomy. If there are super-compact objects which possess unstable circular
orbits of photons, however, it may be difficult to distinguish them from black
holes by observing photons. As a model of super-compact objects, we consider a
gravastar (gravitational-vacuum-star) which was originally proposed by Mazur
and Mottola. For definiteness, we adopt a spherical thin-shell model of a
gravastar developed by Visser and Wiltshire, which connects interior de-Sitter
geometry and exterior Schwarzschild geometry. We find that unstable circular
orbits of photons can appear around the gravastar. Then, we investigate the
optical images of the gravastar possessing unstable circular orbits, with
assuming the optically transparent surface of it and two types of optical
sources behind the gravastar: (i) an infinite optical plane and (ii) a
companion star. The main feature of the image of (i) is that a bright disk and
a dark thick ring surrounding the disk appear in the center of the region which
would be completely dark if the compact object was not the gravastar but
Schwarzschild black hole. Also in the case (ii), a small disk and arcs around
the disk appear in the region which would be completely dark for the lensing
image by Schwarzschild black hole. Because characteristic images appear inside
the gravastar in both cases, we could tell the difference between a black hole
and a gravastar with high-resolution VLBI observations near future.
