To unlock the For Posterity trophy in The Division 2, you will first need to be in a group of four agents. You can do this either with friends when playing the game in co-op, or you can do it via matchmaking if you don't have people to play with.
To enter photo mode, open the main menu using
> hold
for photo mode.
To take a photo with all 4 agents in the frame, the best place is either during a mission when you are all in close proximity, or at the end of a mission when you are all stood together.