We compute prompt photon production from gluon fusion in the presence of a magnetic field in semi-central relativistic heavy-ion collisions. The main ingredient is the treatment of the high gluon density at early stages of the collision where intense magnetic fields are also present. The magnetic field opens new channels for photon production that are forbidden otherwise. The elliptic flow coefficient $v_2$ is also computed. The calculation takes into account the saturation scale and phenomenological factors. Overall, the treatment gives a good description of the excess photon yield and $v_2$, particularly at low values of photon transverse momentum.