  We compute the two-loop BSM contributions to the $h\longrightarrow
\gamma\gamma$ decay width in the SM extended with a real triplet of $SU(2)$. We
consider scenarios in which the neutral components of doublet and triplet do
not mix, so that the lighter neutral scalar $h$ has (at least approximately)
SM-like couplings to fermions and gauge bosons. We focus on the two-loop
corrections controlled by the quartic scalar couplings, and obtain explicit and
compact formulas for the $h \gamma \gamma$ amplitude by means of a low-energy
theorem that connects it to the derivative of the photon self-energy w.r.t. the
Higgs field. We briefly discuss the numerical impact of the newly-computed
contributions, showing that they may be required for a precise determination of
$\Gamma[h\rightarrow \gamma \gamma]$ in scenarios where the quartic scalar
couplings are large.
