Concealing Canvas

Somebody's hidden a safe, or other valuable object, behind a painting on the wall. In a common joke, the safe is hidden behind a painting of a safe.

To be fair, a painting is probably the most obvious object to hide a safe behind; on the other hand, people rarely seem to consider installing safes behind other common objects hanging on walls, such as mirrors and tapestries.

Sometimes the painting will suggest the presence of the safe, the nature of the valuables or perhaps even be a clue to the safe's code.

This is ridiculously common in Interactive Fiction.

Film

 * Biff's office in Back to The Future Part II.
 * Wallace & Gromit had this. Wallace hides his piggy bank in a safe, behind a picture of a piggy bank.
 * Entrapment does this.
 * Tomorrow Never Dies also does this.
 * Thunderball has a secret map behind a painting, showing top secret locations.
 * National Treasure: It wasn't a safe, and it wasn't a painting, but it's a similar idea....
 * There's a safe behind a painting of Hitler in the movie Valkyrie.
 * Mazzini's family tree, on which he records who he has to kill to become Duke in Kind Hearts and Coronets, is taped to the back of a painting.
 * Sherlock Holmes. Sherlock quickly checks the safe behind his painting is still locked after waking up to find Irene Adler in the room.
 * The Pink Panther has a double-ended safe behind two paintings.

Literature

 * The first Artemis Fowl plays with this: it has a safe hidden behind a painting... which is a decoy for the safe built into the frame of the painting.
 * In typical Frederick Forsyth fashion, The Fourth Protocol meticulously details the locating and destruction of a safe hidden behind a painting.
 * In The Three Investigators volume The Case of the Weeping Coffin, the eccentric millionaire's house is littered with these, to the point that the curtain that doesn't conceal a safe is interesting.

Live Action TV

 * In season 3 of Heroes,.
 * An episode of The Monkees had a safe behind a painting of a safe behind a painting. That is, the "regular" painting was removed from the wall to show a painting of a safe, which was removed to show a real safe.
 * Lost: Ben Linus keeps all his information on Charles Widmore in a wall safe behind a painting.
 * On Leverage, we learn Hardison has hidden large quantities of cash inside the frame of his painting of Harlan Leverage III (AKA Old Nate).
 * Played with on Burn Notice.
 * In one episode, an art dealer has hung a stolen painting on the wall and covered it with another, larger, painting.
 * In another, an assassin hides a disc of sensitive data inside the wall - concealing the hiding place with a layer of plaster.
 * In a few episodes it's noted that it's foolish to hide things in those places and suggested that the best places to hide small things of value are behind door hinges (unscrewed and hollowed out) and that the best places to hide bugs are behind receptacle covers because no one looks there and there is a steady source of power.
 * Batman.
 * In the episode "The Duo is Slumming" the plans for an airplane are in a wall safe concealed by a painting.
 * In the episode "That Darn Catwoman" stately Wayne Manor has a wall safe hidden behind a painting.
 * At least once in Alias: Vaughn goes to Virginia to his wife's mother's house and has to steal a code. The code is in a safe on the wall behind a painting, and the painting even has hinges on the right side attaching it to the wall and facilitating access to the safe.
 * Doctor Who had variations on this from time to time. In "The War Games," for example, a painting in the World War I general's quarters concealed his SIDRAT (a knockoff TARDIS) controls. In "The Five Doctors, a door could only be opened by playing a tune on Rassilon's harp. A nearby painting of Rassilon playing that harp showed the tune.
 * In Charmed, Cole hides a safe behind a portrait in his penthouse apartment when he's It doesn't have anything in it except a posthumous note to his wife, though.

Radio

 * Adventures in Odyssey used this trope, and the stock joke: A safe was hidden behind a picture of a safe.

Tabletop RPG

 * Call of Cthulhu (tabletop game) adventure Utatti Asfet has two wall safes behind paintings: one behind a lithograph of the Mississippi paddle wheeler "The Bayou Queen", and one behind a portrait of Father Michael Thibidoux, a Cthulhuoid cultist.

Truth in Television

 * Still used in Real Life. Model 20720 comes with a built-in frame to conceal the door, and a remote control. This trope is mentioned by this seller and this one.

Video Games

 * 2027: Your safehouse in Paris has one.
 * A few apartments in Deus Ex Human Revolution have safes hidden behind paintings, Adam Jenson has an emergency stash hidden behind his TV which slides up on the wall (It contains a pistol, some ammo, and a bit of cash, along with some non-interactive medicine).
 * One of the first levels of the first Splinter Cell featured this.
 * This is the first place to look for safes when you're a thief in the Quest for Glory games. The fourth game hangs a lampshade on this in the Thieves' Manual, explaining that people still do it despite the trope being well-known because they are stupid.
 * Dr. Fred's safe in Maniac Mansion and Day of the Tentacle is hidden behind a portrait of him, with hair.
 * In the third Sly Cooper game, not only are safes hidden behind paintings, but the combinations are hidden in the paintings.
 * Pokémon Red and Blue - the switch to open a secret door lies behind a poster in the Celadon Game Corner.
 * In Covert Action safes are obvious, but in some objects the Player Character can hide bugs in break-in mode, and painting on the wall is one of them: turn it, plant a bug, turn it back. Also you see the guards walking around sometimes quickly open and close those paintings, presumably to check for bugs some "inventive" spy always may leave behind.
 * Used in the Baldur's Gate series of CRPGs, mainly by inferring that there are secret compartments behind clickable paintings. Subverted once in Shadows of Amn, however: attempting to pick a locked door in a thieves' guild would reveal that it's just an incredibly elaborate painting with a doorknob attached.
 * It's probably easier to list the Room Escape games that don't use this trope.
 * Medal Of Honor: Frontline: there is a villa, inside the villa is a map hidden behind a painting and documents in a cuckoo clock. There are controller breaking number of cuckoo clocks and paintings AND WHAT WERE YOU DEVELOPERS THINKING
 * Morrowind: inside the Redoran manors in Ald'rhun a door is hidden behind tapestries.
 * In Ace Attorney Investigations, each prosecutor's office contains a wall safe meant to store evidence needed for trials. Edgeworth hides his behind the suit jacket and vest he keeps framed and hanging on his wall.

Web Comics

 * Problem Sleuth has a few of these, subverted in every possible way. Early on, we see that Problem Sleuth's safe is just a blank facade, concealing a painting. Not to mention the safe hidden behind his window. It Makes Sense in Context.

Western Animation

 * Bender's Big Score has the safe behind a painting of the Professor opening the painting to find the safe.
 * An episode of The Simpsons had this in Burns' mansion. Bart, being genre savvy, searches for it, only to find a Beefeater guard as well.
 * Non-safe (unsafe?) example: Vlad Masters of Danny Phantom hides a portal to the ghost zone behind a painting of himself.
 * On the other hand, the Ghost Zone side of the portal is "concealed" behind a giant football.
 * Darkwing Duck hides a safe behind a family portrait at home. He also has another of these, holding his case files, in his secret lair.