Little bit irritating this one was. I just got it and the drop rate on the bar brawl mission was abysmal. Got 1 in 25 attempts on tier 3 rematch!
Usually (but not always) only bad ass enemies drop the pictures and there are lots more of them when you fight the enemies in the bar WITHOUT starting the actual bar brawl mission, also the enemies almost spawn indefinitely when doing this. I managed to get the remaining 4 I needed in about 10 mins this way. Worth a try for anyone struggling.