Red Superfly
You creepy little stalker
Apart from saying the "it's just a movie" thing, it's also BTTF's own made up rules of Time Travel, made for dramatic effect, rather than scientific accuracy.
The picture that Marty had, and himself, disappeared bit by bit as the time-space continuum was shifting itself to accomodate the fact that Marty has jeopardised his own existance, as opposed to his environment in Part II. It's obviously down to the fact that he is the one travelling through time that causes such diverse results, and that travelling through an alterred timeline of one's history/future, is perfectly fine in BTTF-land, as long as they don't jeapoardise their own existance.
There are many things that don't make sense, like where was Marty's duplicate from the Biff-ruled future in BTTF Pt 2? It's just one of those things that shouldn't be thought too hard about, because, like I said, BTTF's rules are self-contained rules utilised for maximum dramatic effect. They are science-fiction-fantasy films.
As for his parents not rememberring Marty, I am the spitting image of one of my uncles when he was my age, and only until they saw a photograph of my uncle when he was my age did my family realise how much I looked like him.
The idea that his parents not rememberring what Marty looked like, and the fact they never had any pictures of Marty while he was in the 50's, mean it's understandable they'd forget. Also, the idea that their son travelled back in time to play a gig at their high school would be the last thing they'd ever think of.