Though Wonder Woman is an origin story, largely set during World War I, the movie begins with a modern-day Diana Prince (Gal Gadot) receiving a photograph in the mail. She sits down at her Paris office to examine the black-and-white picture. It shows her in full Wonder Woman regalia standing with Steve Trevor (Chris Pine) and the rest of their cohort in what appears to be a war-torn city. On the back is a note from Bruce Wayne, Batman himself.
This photograph, which originally appeared in 2016's Batman v Superman: Dawn of Justice, is the only link between Wonder Woman and the rest of the DC Extended Universe. It's a brief interlude, but it still may leave audience members who skipped last summer's face-off between the Dark Knight and the Man of Steel feeling a little lost.
Wonder Woman made her debut in that movie, first as a mysterious antiquities dealer who shows up at a party thrown by villain Lex Luthor. Her goal is to steal a drive full of information that Luthor has gathered about all the "metahumans" in the universe, including Superman, Wonder Woman, the Flash and Aquaman. Batman eventually gets a hold of this drive and finds the very old picture of Wonder Woman on it, leading him to believe that PRince is immortal and some sort of superhero in hiding.
We find out the origins of the photo in Wonder Woman after Diana crosses no man's land to take back a town invaded by the Germans. Trevor and the other soldiers help her take out German troops and destroy a clock tower from where a German sniper was picking off the Allies. The tower lays fallen in the background of the picture.
Wayne apparently dug up the original to send to Diana after finding it on the flash drive. Perhaps it's his way of luring her into joining the Justice League by reminding her of the good she can do as part of a team. Or perhaps Wayne is making a romantic overture (surely unaware that her true love, Steve Trevor, is in the photo.)