Although Patty Jenkins did something truly historic wth Diana Prince's first solo movie, Zack Snyder still holds the distinction of being the first modern filmmaker to include Wonder Woman in a film. The Batman V Superman director played a massive part in getting the DCEU off of the ground, and that influence extended to playing a significant role behind the scenes on Wonder Woman. This influence apparently even included a hidden Snyder cameo in the background of Diana's Veld photograph -- which was taken on the first day of Wonder Woman's production. Deborah Snyder explained:
...it was an exciting first day of shooting since it was a passing of the torch. The photo links Batman V Superman: Dawn of Justice to Wonder Woman, so both Zack and Patty were present that day, working collaboratively. It was an exciting shared moment as we watched the DC world expanding and it was wonderful to be able to capture that moment in such a unique way. Photographer Stephen Berkman must have shot twenty glass plates before he, Zack and Patty all felt they had the winning photo, as shooting a daguerrotype is art wrapped in a science experiment.
The mysterious black and white photo of Diana from the aftermath of the Veld sequence is easily Wonder Woman's most substantial connective thread to the rest of the DCEU. Because the picture factors so prominently into the events of Batman V Superman: Dawn of Justice, it was taken on the first day of production for Diana's solo movie, with Zack Snyder and Patty Jenkins actively collaborating on the creation of the old timey photo. That was his primary purpose for being on set that day, but (ever the self-respecting geek) Snyder ultimately opted to throw on an American WWI uniform and hop into the background of the picture as well -- forever solidifying his place in Wonder Woman's corner of the DC universe.
Of course, his actual cameo is so minor that you would likely never pick up on it without being told to look for him. Check out the background of the photo below for a closer look at Snyder's cameo.
Despite how cool it is to see Zack Snyder show up for a Wonder Woman cameo, it doesn't sound like you should bother looking for him in other portions of the Veld battle. As pointed out in the newly released book Wonder Woman: The Art and Making of The Film (via Screen Rant), the rest of the Veld scenes were filmed considerably later in Wonder Woman's production. Snyder is in the background of the black and white photograph, but your chances of seeing him lurking in trenches or charging No Man's Land seem slim.
As the photo also shows up in Dawn of Justice, this means that Zack Snyder has two notable cameos in his Batman/Superman adventure from last year. In addition to showing up in the background of this photo, Snyder also used his hands to double for Bruce Wayne's hands in a close-up shot. This is hardly surprising, as Snyder has always loved to sneak himself into his projects, but now he has even found ways to insert himself into the projects of other DC directors as well.