Painting

Paintings in Minecraft are simple, low-resolution versions of a canvas; they currently appear in Beta, Alpha, Infdev, Indev, and the official version. Most of the twenty-six (19 in indev and infdev) current canvases are by Kristoffer Zetterstrand and are shrunken to a low resolution to fit Minecraft's graphical theme. Five more are edits or new paintings. A random image from among these is chosen each time a painting is placed. Paintings are currently nonflammable and protect covered flammable blocks from catching on fire.

Paintings can only be placed on flat, vertical surfaces. An easy way to get a painting to fill up the area you want is to mark the bounds with any solid block and place it in the bottom-left corner. It expands canvas size to the top-right to fill up the wall the best it can. Going up and right one block increases the chance for paintings 4x2 or bigger, though it may still take a few tries to get a large painting. As revealed in the en_us.lang file, Paintings are entities, not blocks.

If hit with a throw-able item (such as an Chicken Egg, a Snowball, an Arrow, or a Fishing Rod Bobber), the painting will drop from the wall and can be picked up and placed again. Arrows will disappear when they hit a painting. Lightning will also knock down paintings.

As of 1.8 players can hit a critical hit on a painting by hitting the painting whilst jumping.

Crafting
Paintings can be crafted with any color of wool. The color of Wool used does not have an influence of the picture chosen when the painting is placed.

As of Beta 1.2, paintings will drop when any supporting block is removed, making the creation of "Secret Doors" more difficult. You can still attach a larger painting to a block beside a door, and it will cover the door, making it hidden. Note that ladders, signs, and pressure plates technically count as full blocks, but only signs, doors, and pressure plates can be used to aid in the creation of secret passages. Also, trying to open a door behind a painting may knock the painting off the wall.

Paintings may additionally be used to hide chests in walls, as the painting will not prevent a player from opening the chest. This is especially useful on large multi-player servers.

Paintings are not technically a block, and thus can simultaneously exist with water and torches in the same space as itself. An example of this is: place a torch on the wall, place a painting on the same block of the wall by clicking on the wall (not the torch). If the painting is 1x1, it cannot be removed without first removing the torch (This was fixed in Beta 1.8). Larger paintings on walls covered with torches can still be removed by left clicking. Players and mobs are able to walk through paintings, and light shows through paintings as well.

The lower left edge of a painting is not always the block you put the painting on, some of the larger paintings extend down and to the left of the block you placed the painting.

If rain hits a painting, it may be blackened.

Canvases
There currently are 26 paintings in version 1.2.5. These are mostly based on paintings by Kristoffer Zetterstrand, who also created the Minecraft versions.

Bugs

 * Paintings placed facing the east/west direction are darker than paintings placed facing the north/south direction.
 * A painting placed over a light source (like a Jack-O-Lantern) will allow the painting to act as a lamp-shade, lighting the room.
 * A painting can have a sign placed in the same block space (but on a different face), this is because a painting is an entity, so it technically does not take up the block it is in. 
 * Paintings do not use the new Smooth Lighting option. When using Smooth Lighting, the painting will use the old lighting, despite the blocks around it. This may lead to the painting accidentally turning completely black, and you will have to place it somewhere else.
 * If you have your back against a painting and you fire an arrow that painting will come off of the wall and the arrow is destroyed. An arrow is also destroyed when you fire it at a painting normally.
 * If you place a fence on a block, then put a painting on the block, it will sometimes select a 1x2 painting. This painting cannot be removed without removing the block or fence. A way around this is to place the painting before the fence.
 * If a painting is placed in The Nether, standing directly faced towards a painting within reachable distance may sometimes make the inventory's hotbar look more white.
 * You can break paintings in spawn-protected regions in multiplayer.
 * If you place a glass block (or presumably any other block where torches cannot be placed) so it forms a corner in front of the painting's edge, then attempt to place a torch on the side of the glass block facing the painting, the torch will be placed on the painting.
 * When sleeping in your bed, right clicking the screen with a painting in your hands will throw the painting.
 * There is a bug that allows you to place a torch on a painting if you hold down the right mouse button and roll across the painting. Alternatively, paintings can be placed over torches.
 * As of 1.8, paintings with any blocks directly above them will appear totally dark, regardless of how much light is available (unless the blocks are transparent).
 * In 1.8, it is possible for the player to critical hit a painting by jumping and hitting on the way down. (This was fixed in the 1.9 Pre-release 5 where all paintings were never shaded in a dark color due to the glitch)
 * When a painting is on a block above another block when the painting is destroyed it may go in the blocks and rest on the lower block.Note that it is retrievable.
 * When using the "pick block" function in creative mode, you receive a bedrock rather than a painting in your inventory.(As of 1.2.5 full release)

Trivia
pig's face.
 * To hide a secret room, placing a painting over an open area prevents the doorway from showing, though can still be walked through.
 * NPC Villagers however, can interact with doors hidden by paintings, easily giving away the secret.
 * On 26 April 2011 Notch stated that custom paintings could feature in a future update.
 * Some reports indicate that if a creeper explosion destroys walls with paintings on them, the paintings will fall off the walls and are able to be retrieved.
 * They used to be indestructible in SMP, only dropping "fake" paintings every hit, which could not be destroyed or picked up. This was fixed in Beta 1.2.
 * When a painting was placed, it sometimes showed a four by four area of solid purple with darker purple between each block. This was because Notch forgot to update the painting texture for the Beta 1.2_01 build, when he added a new painting. This was fixed in Beta 1.3.
 * In SMP, if a player is standing behind a painting, other players may not see their name.
 * If a block is neighbouring a painting, the painting will appear almost totally black.
 * Paintings' lighting being fixed is listed twice in the updates for the minecraft news.
 * Paintings are not actually attached to the wall. If you look closely, there is actually a space between the painting and the wall.
 * The texture on the back of a painting is almost the same texture of wooden planks. (It is darkened slightly.)
 * Prior to Beta 1.3, a painting similar to "RGB" existed, depicting the Mona Lisa, albeit with a
 * Paintings are not a block, but an entity. A simple way to test the is by placing them on a cactus.