Rather than set Wonder Woman in the present day as most superhero movies usually are, Warner Bros and the DC Extended Universe decided to turn the clock back a century and tell Diana of Themsycira's origin story during World War I. It was one of the many ways that this movie differentiated itself from most of the competition, and because it was set in the past, it did a better job of functioning as a standalone tale compared to Batman v Superman: Dawn of Justice and Suicide Squad. Nevertheless, Wonder Woman didn't skate by without throwing in a few ties to the wider DCEU, and it was the "bookends" of the movie that helped set up her appearance in Justice League later this year.
At the beginning of Wonder Woman, we saw Diana going to her day job at the Louvre, where she studies and analyzes antiques. While in her office, she received a delivery from Wayne Enterprises; the photograph of her from World War I standing with Steve Trevor, Sameer, Charlie and Chief. As you'll recall if you've seen Batman v Superman: Dawn of Justice, Diana was investigating Lex Luthor because he possessed this photograph, hence how he knew she was something "special," i.e. one of Earth's metahumans. Bruce later found a digital copy of this picture, and he was understandably shocked to learn that the woman he had just met was alive 100 years earlier and looked exactly the same.
The picture wasn't mentioned again in Batman v Superman: Dawn of Justice, but sometime after that movie, Bruce Wayne found the physical copy and had it sent to Diana in Paris. With Lex Luthor now in jail (or Arkham Asylum, if you watched Batman v Superman's Ultimate Edition) and federal agents seen confiscating his belongings at the end of the second DCEU entry, it must have been easier for him to obtain it, be it in his civilian guise or as Batman, than if Luthor still had all his resources. In any case, the photograph is back in Diana's hands, with Bruce using it as a way to try to strengthen their new relationship.
The photograph by itself was certainly a big deal, because without it, Diana wouldn't reminisce about the past, and we wouldn't see what her first time in "man's world" was like. But let's not count out the importance of the feeling that Diana had when looking the picture. After thanking Bruce Wayne over email and recollecting on her brief time with Steve Trevor, the Amazon decided to continue her mission to protect all life, and Wonder Woman ended with her donning her costume and jumping out to parts unknown. Even though Wonder Woman aided Batman and Superman with fighting Doomsday, this ending clearly showed that she had embraced her heroic legacy again for good.
Batman decided at the end of Batman v Superman: Dawn of Justice to find other metahumans to help him protect the world in Superman's absence, specifically with a mysterious threat he believed was coming (and we saw him continuing his search at the end of Suicide Squad). Although by that point Diana was friendly with Bruce, it was unclear whether or not she was on board with his plan. The Justice League trailers have since shown us that she's decided to work with Bruce to recruit other superheroes, but thanks to Wonder Woman, we know what pushed her to help the Caped Crusader. By reflecting on her past, she was able to move her future in the right direction. Now she'll be be instrumental in protecting humanity when Steppenwolf and his Parademon army invade Earth to search for the three Mother Boxes.
Wonder Woman is now playing in theaters, and you can see Diana fight alongside Batman, Aquaman, Flash, Cyborg and Superman when Justice League is released on November 17.
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