While no context was provided in Batman v Superman: Dawn of Justice for how Diana ended up in that photograph during World War I, Wonder Woman revealed that it was taken after Diana and her allies successfully liberated the Belgian village of Veld. Diana had already spent some time with Steve Trevor, Charlie, Sameer and Chief, but when the Amazon charged across No Man's Land to take the brunt of the German firepower, they aided her on the battlefield and within the village, subsequently leading the rest of the Allied Forces to join them. After Veld's liberation, the four men remained with Diana, and while Steve Trevor sacrificed his life to prevent Doctor Poison's gas from being delivered, the rest of them lived to see the Great War conclude. A century later, Diana received the physical photograph from Bruce Wayne, and you could tell by her expression that it was an important memento for her.