It's too large to be Venus or Mercury, never even mind the other planets, which are much further away when you see them during the day.
Supernovas can be such huge light sources that they can clearly be seen, even during the day, so that could be an option.
However, there's a small dark spot in the middle, which is weird. So it must be closer by. My guess would be someone hanging from a parachute (white sails, photographed directly from below). The central dot would then be the person hanging from the parachute.
Alternatively: