I think it's a pic of tara with her gf/ one night stand girl of whom she tells us about.
I generally after 4 updates use a save from this website to unlock all gallery so if i wanted to unlock this pic i would have to start the whole game over as i dont know the choices the player use save i used have made.
But my guess is about 3 updates before when we are asked by tara which pic do we choose the smile pic could be trigger. So smile pic, then in this update express concern when ashley tells us to include tara for boobs comparison and let tara herself send us the pic then after that when she tells us about the one night stand just promise her to not tell anyone (dont say that she should tell ashley and laura, just say you wont say anything to anyone about it.) After that when tara is on couch say you wont sneak on her just respect her wishes. I think these choices might trigger the pic 514, ofcourse i can be wrong.
Has anyone tried this path here or maybe
Le Stag can hint about the path i said is correct or not.