CsPbI3 is of high interest for photovoltaic applications owing to its low bandgap, provided it could be stabilized in its γ perovskite phase. Instead, it energetically prefers to adopt a so-called yellow, δ phase, with much larger bandgap and reduced photovoltaic properties. Here, using an original constrained Density Functional Theory method, we mimic the effect of thermalized photoexcited carriers, and show that larger concentrations in photo-excited carriers (i.e. larger optical pump fluences) effectively reduce the energy difference between the δ non-perovskite ground state and the γ perovskite phase. Even further, the stability of the phases could be potentially reversed and therefore the γ phase stabilized under strong illumination. We also report large photostriction, i.e., large photo-induced strain for all phases in this material, making CsPbI3 suitable for other applications such as photo-driven relays and photoactuators.