The photo makes no sense
So, the tombstone keeps changing in the photo, but then the whole tombstone just disappears.
In a childish logic, it makes sense.
In adult logic, not so much.
Why would Marty and Doc go to the graveyard in 1955 (in the middle of the night, too!) and take a photo of an EMPTY PATCH of grass? What kind of sense does THAT make? Can anyone explain why they would ever DO that?
Shouldn't the photo change to something else completely, OR disappear completely? Why does the tombstone disappear, but not the photo? Why are time travel phenomena so selective?
In any case, it makes no sense.
The REASON why they wasted an expensive light bulb and film to painstakingly develop later on, was because it was a SIGNIFIGANT thing they were photographing! IT WAS IMPORTANT to take that photo! It had crucial information they wanted to save.
It was not easy to take photos in the fifties, you needed big, cumbersome light bulbs, lots of manual adjustment, tinkering, etc. and frankly, they were lucky to get such a good result of the tombstone.
I didn't even mention the film purchasing, installing, development and all that process.
WHY WOULD THEY WASTE ALL THIS TROUBLE JUST TO TAKE A PHOTO OF GRASS AT NIGHT?!
You see how this makes NO sense, don't you?