system
Given the following post, respond as if you're commenting on Reddit:
human
[Back to the Future] Why do Marty's siblings gradually fade from the photograph instead of it being instantaneous?

Is this implying that there is a grace period where two futures can exist while a circumstance is being decided on by a subject (eg Marty's parents)? Are there only a finite set of futures to choose from and this gradual fading is the slow decision from one exact future to another?  

But then why does the photograph remain the same even though Marty's siblings are more successful in the future where George punches Biff?

With the butterfly effect accounting for all kinds of variables being minutely interfered with, is it reasonable to say that there are certain facets of destiny at hand and each exchange of a time-traveling subject pushes these specific variations of the future toward one end or another?
gpt
Time travel is all about quantum mechanics, which is all about probability.  It's not a black/white thing.  As the timelines diverged more and more the probability that they would not exist increased which caused the probability that the photons which created the image on the picture would not bounce off of them, causing them to become more and more transparent.