came up with a theory that explains how new york blows up, and how everything in issac's/sylar's paintings end up happening in the present timeline.
to start with, i think as some do that there are several timelines in the show.
timeline 1: claire is killed at homecoming by sylar, becomes practically immortal, new york blows up (not much else is known).
timeline 2: claire does not die at homecoming (peter or someone else saves her), new york blows up, sylar becomes president. this is the future we saw in last week's show. it's known that some things issac paints in this timeline doesn't happen in the timeline.
timeline 3: our present timeline where the show takes place.
timeline 4: timeline where new york doesn't blow up.
we know that in the show, issac has not been wrong on a single painting yet (at least in timeline 3), so pretty much it's going to happen, new york will blow up and will rebuild in 5 years. i remember watching another show, and in this show, they were talking about how comic book writers had a window to another dimension and what was in the comic was actually happening in that dimension. i think that sort of applies here. issac in the other timelines is perhaps only able to look at the future at timeline 3, the present timeline, and anything else that he draws in the other timelines is invalid. so everything he draws here will come true, as this could have been the timeline of destiny or something.
also notice how it seems like every time hiro makes a change in the past, it always moves him into another timeline of some sort. it's only when he doesn't make changes to the past when the timeline stays the same (i.e. charlie when he couldn't change her past). his power is probably limited to where he can't stay in the current timeline if he makes major changes to it, and we get into more complex stuff. an alternate version is that a different sequence of events happen in the other timelines completely without hiro's meddling, and hiro really can't do much to change the past, as it was supposed to happen this way based on issac's paintings.
peter's dream sequence of doom enters this as well. notice how it looks like nobody was in the city when peter exploded? and the fact there were two peters? peter was seeing the dream in his own eyes (or sylar's) while peter was blowing up? what if hiro stopped time long enough to get at least some people out of there, so that the death toll isn't as high as timeline #2. the things that happen in the painting occur anyway, but a different sequence of events happen here leading to the rebuilt new york.
so that's how everything is going to turn out. if new york doesn't blow up, then the show has probably moved out of timeline #3 and into timeline #4, and we have a bunch of new things to think about.