IMG archives are used in all GTA III era games to store model and texture files, using the .img extension. There are several IMG files in GTA San Andreas and only one in GTA III and GTA Vice City - together they contain the DFF models for most solid objects in the games, as well as TXD texture files which hold the textures for the models.
IMG archives work much the same as ZIP or RAR files, although they cannot be opened by Windows itself or any common ZIP/RAR applications. Instead, a specially-designed IMG extractor must be used - the most common of which is IMG Tool. Some other applications designed to install GTA modifications can also access IMG archives, such as the GTA Garage Mod Manager.
The most notable archive in all three PC games is gta3.img - this archive contains the DFF and TXD files for all buildings and vehicles in the game. To modify the appearance of a vehicle or building, either the model or texture file must be replaced inside the gta3.img archive in the "models" directory. The name of a vehicle's DFF and TXD file is determined by its internal name, such as 'blistac' for the Blista Compact.