There's actually two photons involved, not just one: http://en.wikipedia.org/wiki/Electron%E2%80%93positron_annihilation http://en.wikipedia.org/wiki/Electron%E

There's actually two photons involved, not just one: http://en.wikipedia.org/wiki/Electron%E2%80%93positron_annihilation http://en.wikipedia.org/wiki/Electron%E2%80%93positron_annih... (If there were just one produced by annihilation, you'd have great difficultly satisfying conservation of momentum and having that photon move at the speed of c (when looking in the center-of-mass frame).)