Power of two generally doesn't have anything to do with rendering/processing performace, but how much video memory it takes up when loaded, as well as for overall project download size because of spritesheeting.
I'm no expert though, and I can't seem to find the documentation I recall reading about this previously. Maybe it was a forum post. Someone else may be answer with more detail, but the general advice will be don't worry about it, and you should avoid large images as much as possible and use tiled backgrounds instead to manage video memory efficiently.