Skins

Skins are, in the Grand Theft Auto games, textures that are placed over a blank three-dimensional model (a DFF file) used for clothing, vehicles, weapons, etc. All skins are stored inside an IMG Archive, and a TXD archive, except in Grand Theft Auto: Vice City and Grand Theft Auto: San Andreas, where only the TXD archives are present. Not all are the same. While most skins follow the same setup, others don't, because the model they were put on doesn't share the same unwrap. These require the corresponding DFF file in order to be rendered properly in the game; for example the NPC characters in-game some of which are 128x128, 64x64, or anything under 256x256, don't follow the design of the user skins.

User-defined skins
A feature in the PC versions of Grand Theft Auto III, and Grand Theft Auto: Vice City was letting the player create their own skins. These skins have a different layout in each game. All skins created by the user have to be a 24-bit Bitmap (bmp) file, and have to be 256 by 256 in size. The parts of a skin are mirrored symmetrically around the model so the image creator would only have to create half of the body part. These are usually placed in the skins folder in the install directory, or wherever the games were installed. While skins in GTA III can be accessed at any time other then when the player has the prison jumpsuit on, skins in GTA Vice City only work with the street outfit.

Texture guides
Below is the design template from GTA Vice City which the player must follow for the skin to come out properly: