as far as I know the power of 2 is to make your comp more efficient! can use odd sized textures but will use more memory and be a likkle slower (depends on comp etc)

they should be SQUARE uv space only goes 0 to1 (have seen templates that ARE not square but internally they are square beware of that lol )

so for your Q can push your comp a lot harder for a single pic need more speed for any animations hope that's helped