Photos Lie

"A photograph is a secret about a secret. The more it tells you, the less you know."

—Diane Arbus

Family portraits generally all look the same. The family all standing together, smiling and happy, hugging each other and wearing ugly Christmas sweaters. However, the photo doesn't show what happens before and after you take it, which is sometimes the most important part. Usually comes in two varieties.

Played for Laughs, you'll see a scene of everyone arguing and yelling, but they stop instantly to pose when someone takes out camera and usually start arguing again once the picture is taken.

Played for Drama, the photo is usually shown first and there doesn't seem like there's anything wrong until someone reveals the story of what happened before or after it was taken.

Examples of Photos Lie include:

Played for Laughs

In Malcolm in the Middle, everybody's arguing as usual. As soon as Lois counts to 3, they instantly stop and strike a perfect pose. Once the picture's taken, it's back to Shouty Town.
Averted on Soap, where the camera catches them in mid-anarchy.
3rd Rock from the Sun did the arguing family version. Everyone was posed smiling, but then Dick, no longer able to tolerate the shallowness, let off a Big No the split the second the camera went off, ruining the picture.
There's a Microsoft ad showing a mom taking family pictures, and there's not a single picture with all of the members behaving at once. So she uses the advertised tool to edit them into one perfect picture.
When the baby is born on The Fresh Prince of Bel-Air, the whole family crowds around the hospital bed to argue about what to name him. When the doctor offers to take a photo, they all stop and smile, then immediately start to argue again after.
On How I Met Your Mother, Lily goes out of her way to keep Ted's date out of her birthday photo. Ted later calls her out on how forced and faked her birthday photos are.
In Gunnerkrigg Court, when we see an old photo of Surma as a young teen, she looks nearly identical to her daughter, Antimony, down to her deadpan facial expression. Several chapters later, a flashback to the day the picture was taken shows Surma complaining about how serious she looks in the picture, and quickly makes it clear that her personality was quite different from her daughter's.

Surma: I bet it looks stupid! I always look so serious in photos! I bet I had on me serious face!

Played for Drama

On an episode of Burn Notice, Madelyn gets out an old Christmas photo from Michael's childhood. It seems happy enough at first, but then she points out Michael's black eye and how he got it.
Zig-zagged in an episode of Royal Pains. Hank's father gives Hank a picture of himself with Hank and Evan when they were young kids, and Evan remembers what a good day that was. Hank disagrees, and flashbacks reveal that the photo was taken after their father used his sons to help him steal back a tractor that had been repossessed. As more flashbacks are shown, though, it turns out that Hank learned some of his valuable present-day skills from that experience, and the three of them bonded over the whole thing: it actually was a good day.
In Watchmen, the Minutemen photograph shown early in the movie seems innocuous enough until later on when a flashback reveals the Comedian attempted to rape Silk Spectre shortly after it was taken.
In Sandra and Woo, when Cloud asks his mother about her childhood in Burma, she looks through her old diary and comments that, while her childhood was apparently quite bad, the pictures she's got from back then gives a much more positive impression.
Judge Dredd: A close inspection of Dredd's childhood photo reveals that the only real part of the photo was Dredd, everything else was composited in because he had no parents.
In Dexter, Dexter has a lot of typical happy-looking family photos with his adoptive family. However, flashbacks show that Dexter hated posing since he didn't feel anything, and was persuaded by his adoptive father Harry to "play the game" and pretend to be a normal person. So, all of his smiles are totally fake in those pictures.
Another episode of How I Met Your Mother uses this trope again. Future Ted shows his kids a photo of brunch with the gang and Ted's family, but reveals that just after that picture was taken, Ted had to confront his dad about him "cheating" on his mom because his parents never told him they were divorced, Robin was irrationally upset at Ted's mom, and Lily and Marshall were trying to seduce each other out of spite. It Makes Sense in Context.
The P!nk song Family Portrait

In our family portrait, we look pretty happy
Let's play pretend, act like it goes naturally

Buffy the Vampire Slayer Season 5 takes this trope just a bit more literally with the photos that have Dawn in them.
Selim Bradley of Fullmetal Alchemist has several family photos where he stands smiling with his parents. The only thing is, he has a different family in each one, because some of the photos are decades old.
Discussed in Criminal Minds. Rossi tells Hotch that it's easy to pretend to be happy for a few seconds, and he has plenty of happy pictures with his ex-wives. Since they're looking at the happy family photo of an abusive husband and the wife who finally snapped and killed him...
Harry Potter and the Deathly Hallows - Part 1 starts with a scene of Hermione performing a memory spell on her parents and editing herself out of each and every photo in their house.