With a new release of Android come new included wallpapers, but since today's Android O release is just a Developer Preview, there's only one for now. If you want to download it in the full 2880x2560 resolution, just click the image embedded below, then save it.

This one depicts light reflecting off Earth's upper atmosphere, but a reverse Google Images search did not yield an original source. I'm definitely a fan of this one, but I think my favorite wallpaper from a Dev Preview is still the Android L background.