BC will only accept powers of 2 textures, so 1, 2, 4, 8, 16, 32, etc, etc... And the most efficient are square textures. Some of the newer cards (ok, any card newer than 3 years) can handle non-power of two textures, but at a price, they will fill up the rest of the texture with wasted memory. On older cards they even fill up a non-square (so instead of a 4x4 or 512x512 you have, say, a 512x64) texture. Or not show it at all.