when you say mass times velocity, I take it you mean relativistic mass? I assume you know all the following, and I'm just saying it to explain my reasoning for

when you say mass times velocity, I take it you mean relativistic mass? I assume you know all the following, and I'm just saying it to explain my reasoning for the above / to elaborate for other readers. One can certainly pick a frame of reference such that the total momentum of the electron and positron afterwards is non-zero, and therefore the total momentum of the photons beforehand (in that reference frame) will also be non-zero , despite photons having (or, believed to have, but I believe it also) 0 rest mass.