Secret Society Group Picture

What is the first thing you do when you get a secret organization of people together, be they heroes or villains? After all, this is supposed to be something that no one else can know about for various reasons, so you can't have matching uniforms, and secret passwords can be cliche.

Of course! You get everyone in one generalized location and throw a party of some sort. Maybe the heroes have some laughter and drinks to celebrate a victory, or the villains get to glower a each other.

Then somehow everyone is called together for a group picture that gives away the connections between every member at a glance, from a glowering glance to a fond smile, and the professional photographer says 'Cheese!'. They even stick around long enough that everyone there can get a copy for their nemesis, children or investigators to find!

These often crop back up at plot points, either as a method of finding the next 'clue' or a hit list by the group's enemies.

Examples of Secret Society Group Picture include:

Anime

One appeared in episode 14 of Tiger and Bunny, showing Barnaby's parents along with several other scientists (including the Mad Scientist who comes to play an important role in the last few episodes of the series) and Barnaby himself as a child.

Comic Books

Every graduating class of X-Men takes such a picture in uniform. While the X-Men do wear uniforms, they are generally considered a secret society because the world at large has no idea who they are, where they come from, and where they go. And they are not a recognized superhero group like The Avengers or the Fantastic Four.
League of Extraordinary Gentlemen: The League that comes together at the beginning of the series doesn't do this (that we see), but their headquarters has a group portrait of an earlier League.

Film

In the film A Series of Unfortunate Events, there was one as well. The children find it while looking through Aunt Josephine's album.
The initial seven agents who dealt with NBE-1 are shown in photographs on Sector 7's walls in the first Transformers movie.
This becomes the huge reveal of the movie The Village.
In Atlantis: The Lost Empire, the expedition team pose for a photograph in front of their spoils. The next scene reveals that the whole thing will be covered up. There is also an earlier photo of the team that found the Shepherd's Journal.

Literature

The Order of the Phoenix in the Harry Potter series has one, though its members were openly against Voldemort; only their location was hidden.
Vitals by Greg Bear has one of these. Late in the story the main characters find an old photo of the evil secret society that's been pursuing them. Cue shocked recognition, as one of the other main characters is in the photo and is revealed to be part of The Conspiracy.
In Robert A. Heinlein's To Sail Beyond the Sunset, a photographer takes a photograph of Maureen Johnson, her daughter, her granddaughter, her great-granddaughter, and her great-great-granddaughter at a family wedding. One of the Howard Family Trustees points out that it could endanger the Howard Families' Masquerade if a photograph of five suspiciously young looking generations of the same family were leaked to the press.
In Lemony Snicket: The Unauthorized Autobiography, members of the V.F.D. pose in a picture during the Second Annual Code-Breaking Picnic.

Live-Action TV

Throughout The X-Files, Mulder found one or two photos of The Syndicate, which also featured his father, motivating his quest for the Truth. This is partly justified by the fact that the group that became the Syndicate existed as a relatively benign government agency well before it turned evil, and all their photos come from that time.
There was also the group picture 'hit list' in Heroes. That one seems to have even been made in duplicate, making one wonder if they were handed out as Christmas gifts that removed all chances of plausible deniability.
The 1974 Dharma Initiative group photo in Lost.
One clue that leads Dexter to the Big Bad and his gang of rapists/serial-killers in season 5 is an old childhood photo showing all of them together, even though some of them denied to know each other. Kind of justified as only someone who knew them anyway could identify everyone as children.

Video Games

Ace Attorney has a few group pictures, where only one is used for this, the picture in Gant's office.
In Nine Hours Nine Persons Nine Doors, there is a group photo depicting the four masterminds behind the first Nonary Game.

Western Animation

The animated series of Men in Black had one, in an episode focused on the founding members of MIB. The photo is used as reference for Jay, who is on the trail of someone who is using time travel to kill the founders before they can get together.
- It's also a photo of the founding itself, since it shows them meeting for the first time and experiencing First Contact.

Real Life

During The Wild West era, one gang decided to get a group picture taken. The copy displayed by the photographer wound up allowing the detective following them to find the gang.