Texture pack

A texture pack is a file that is used to change the in-game textures of blocks, items, mobs and other things. They are usually zip files that have various PNG images in them and a pack.txt. The default resolution for packs is 16×16 (measured pixels in block height and width). Anything larger is called ‘HD’ and requires a patcher to run. Official support for texture packs was added in Alpha 1.2.2.



Installation
To install a texture pack, the file must first be obtained in a ZIP format. Without decompressing the file, place the ZIP file in: This path can be opened by using the Open Texture Pack Folder button from the game client.
 * Windows:
 * Linux:
 * Mac OS X:

Once a texture pack is installed, it can be selected in the Mods and Texture Packs screen accessible from the main menu (but not in-game). The texture pack list can be scrolled by dragging it up or down.

Texture Pack Creation
To create a custom pack, the files you wish to modify must be extracted from minecraft.jar with an archiving or decompression program such as WinRAR. They can then be edited with any image editing program. A good starter choice is the free Paint.net, which handles PNG transparency in an good fashion. Edit each PNG file as desired, using the alpha layer to trim away transparent parts. Save the PNGs as 32 bits, to preserve this transparency layer. When the images are zipped back into archive, names and folder structure must be maintained (with WinRAR, just drag and drop into the right places). For a complete list of files, go to this thread.

List of Modifiable Files
1 Due to a bug, changes to the clock dial and grass/foliage files do not affect their respective items/blocks when installed using the Texture Pack/Mod Menu. Direct installation into the minecraft.jar file can be done instead: open minecraft.jar and the texture pack with WinRAR, then drag and drop the contents of the pack into minecraft.jar. If you mess up, just delete .minecraft/bin and it will reload once you start up Minecraft.exe. After any update, you’ll need to reinstall the pack this way.

High Resolution (32px or more)
 '''These texture packs require a client mod. Therefore, they are not supported by Mojang, the Minecraft Wiki, the Minecraft IRC Channel or the Minecraft Forums .''' For a more complete list of texture packs, see the Texture Pack Central on minecraftforum.net.

Trivia

 * Prior to the Alpha 1.2.2 update, users would need to manually change the texture packs by overwriting their minecraft.jar files. This is still required for HD texture packs.

Resources

 * Large list of texture packs on the Minecraft Forum
 * Second large list of texture packs on the Minecraft Forum