  We demonstrate a single-site and single-atom-resolved fluorescence imaging of
a bosonic Mott insulator of $^7$Li atoms in an optical lattice. The
fluorescence images are obtained by implementing Raman sideband cooling on a
deep two-dimensional square lattice, where we collect scattered photons with a
high numerical aperture objective lens. The square lattice is created by a
folded retro-reflected beam configuration that can reach 2.5~mK lattice depth
from a single laser source. The lattice beam is elliptically focused to have a
large area with deep potential. On average 4,000 photons are collected per atom
during 1~s of the Raman sideband cooling, and the imaging fidelity is over
95$\%$ in the central 80$\times$80 lattice sites. As a first step to study
correlated quantum phases, we present the site-resolved imaging of a Mott
insulator. Tuning the magnetic field near the Feshbach resonance, the
scattering length can be increased to 680$a_B$, and we are able to produce a
large-sized unity filling Mott insulator with 2,000 atoms at low temperature.
Our work provides a stepping stone to further in-depth investigations of
intriguing quantum many-body phases in optical lattices.
