There's another factor, remember. The transient nature of the thing. It only appears in one frame of an (alleged) three frame series. It was almost certainly discovered in that one frame after the fact, and not noticed at the time.Much more likely to be a balloon than a butterfly imo. Could of course be something close to the camera but I'd probably go for it being an artefact of the camera before I would say butterfly. Looks uncannily like a helmet, you can get mylar balloons in this shape.
We could also be looking at a chinese lantern, although an oddly shaped one, perhaps damaged?One shaped in this fashion perhaps?
If this were something he noticed in the sky at the moment, the photographer would have centered the object in the frame. If he were truly a pro-photographer he would have equipment with him. He would have grabbed his best camera with the longest lens. What we've got is one frame, a medium lens, and probably a cell phone camera. How about video instead of a still frame?
Butterflies can pass by unnoticed by the photographer at the moment the photo is being taken. Balloons hang around for a long time. If it were a balloon... how come no more photos? How come no eyewitness report(s)?