I'm surprised no one put up a solution for this so I will -
1. You cannot do this trophy until you are level 8.
2. Once you are level 8 walk up to a zombie press and hold r1+r2, this will grab the zombie and you will take a sort of zombie selfie.
3. Trophy should unlock it didn't unlock straight away for me but I've noticed several trophies taking a couple of minutes to pop any comments/questions feel free to ask please comment before leaving negative as I'd like a chance to sort anything that you feel needs looking at.