Direct photon yield at midrapidity as a function of charged-particle multiplicity \dndeta. The results from this letter are shown as full blue markers and are compared to theoretical calculations for prompt (open green circle) and prompt+thermal (open red square) yields for INEL pp collisions at $\sqrt{s} = 13$ TeV~. The results from the STAR~, PHENIX~ and ALICE~ collaborations are shown as full markers of various colours, and the band shows $N_{\rm coll}$-based extrapolation of pQCD calculations for pp collisions at $\sqrt{s} = 200$ GeV~. The \mbox{Pb--Pb} results are compared with the theoretical prediction by Gale~, which includes prompt, pre-equilibrium and thermal photons. The prompt-photon yield is computed with NLO pQCD using the INCNLO code~ together with the nCTEQ15 nuclear parton distribution function~, and the BFG-II~ fragmentation function.