Can gravitons escape from a black hole?

That's a very good question. Photons are light particles, and you're quite right in saying that they can't escape from black holes. This is why they look black. Gravitons are thought to mediate gravity, and are massless spin-2 particles. If gravitons couldn't escape from a black hole, you'd have a problem because the black hole wouldn't be able to mediate the gravitational force and you wouldn't feel a gravitational force around it, which we believe isn't the case. There are some quite technical reasons for why particles can mediate a force even though they're in a black hole across the event horizon, but it would take me longer than the rest of this show to explain it.