You should roll your pelvis forward and straighten your back. As you roll your pelvis forward, you'll feel your sit bones coming into firmer contact with the saddle. That's what you want. Don't hollow your back either, like the second image, but really no one does that. Your upper arms should make ~90° angle with your torso. Your arms and shoulders should be relaxed, not held in any position. Some riders pull their shoulders up around their ears. Don't do that.
On a long ride, one's position can deteriorate. Every now and again, I'll check my position and all its components. When pilots fly on instruments, they make a constant sweep of their gauges, checking them all in a certain order. That's what you do on a long ride. You constantly check every element of your position and technique.