Check out video below featuring producer Deborah Snyder talking about the Wonder Woman photograph in Batman Vs. Superman as well as adding in the Ezra Miller Flash cameo (watch here).
Regarding The Flash, Deborah Snyder lets it be known they were in the middle of filming Batman vs. Superman and working on the outline of Justice League, when Zack Snyder came up with the idea to add in The Flash in this way.
The Wonder Woman WW1 photograph (watch below) was created especially for Batman Vs. Superman and taken by one of Zack Snyder’s friends. Zack Snyder also made his way into Wonder Woman as an extra.
