Both of them have used up their time-leaps. Chiaki has gone back to the future. Makoto is left in her time. She knows that the only way Chiaki can metaphotically meet her, way after she would have died, is if he gets to see that painting in his own time.
So you are right, by saying that she knows what she will do in the future is to take the right steps to preserve the painting so that one day Chiaki will see it. And when he does, he will know who was responsible for the preservation of the painting.
Here's the conversation from location around 01hr:26min (translated):
Mokoto: The painting... you can see it back in the future. It won't disappear or burn up. It will still be there in your era. I'll make sure of it.
Chiaki: I'm counting on you.