  We demonstrate single-site-resolved fluorescence imaging of ultracold
$^{87}\mathrm{Rb}$ atoms in a triangular optical lattice by employing Raman
sideband cooling. Combining a Raman transition at the D1 line and a photon
scattering through an optical pumping of the D2 line, we obtain images with low
background noise. The Bayesian optimisation of 11 experimental parameters for
fluorescence imaging with Raman sideband cooling enables us to achieve
single-atom detection with a high fidelity of $(96.3 \pm 1.3)$%. Single-atom
and single-site resolved detection in a triangular optical lattice paves the
way for the direct observation of spin correlations or entanglement in
geometrically frustrated systems.
