Alternate theory: it doesn’t take “time” (I understand exactly what you mean) for the change to propagate. Instead, what we are seeing is a representation of the “thought-driven quantum reality” effect. That is, reality is determined by what human consciousness thinks is right. To wit: whether the damned cat is alive or dead doesn’t depend on quantum equations, but on whether you believe it is alive or dead.
And this applies to time travel. As soon as Marty appears in the past, the timeline is threatened. Just being there could (and most certainly did) create any number of small changes that may “ripple” through to the Future, or be dampened by the “weight” of time. The longer he stays in the past, the higher the chances of creating a future different that existed when he left increases. This is manifested by the disappearing of the children, in age order, from his photo. It’s a representation of the quantum state equations. The longer George and Lorraine don’t get together, the higher percentage that the future will be different. But it isn’t different “yet”, until human actions have “locked down” the timeline. There’s still a chance Marty could be born but maybe Dave and/or Linda might not.
When Marty forces the future to lock in, the picture (and his own existence) revert.
What we don’t see is, there should be changes to the photo that even Marty might not notice. (I think the filmmakers missed an opportunity by not changing one of the kids’ shirts). Because, as we see, Marty didn’t restore the original timeline, he sort of patched it. Which is why he has cool parents and a new truck.