Minecraft wiki entry for Terrain-atlas.tga, associated text: terrain-atlas_mip0.tga is a 256×128-pixel, terrain-atlas_mip1.tga 128×64-pixel, terrain-atlas_mip2.tga 64×32-pixel and terrain-atlas_mip3.tga 32×16-pixel files that is split into different 8×8, 4x4, 2x2 and 1x1 squares which correspond to the mipmap texture of each block found in-game.