Are these pictures from similar minutes and seconds during the fight? If they are from the same time, I reckon picture on the bottom shows that it is on the beltline because it is the best angle from where everyone can see that the boxing gloves of Daniel Dubios has touched the midsection of Usyk.
It is supposed to be the same punch at the same time from different television camera angles without any time between them.
Low blows can also lift the crotch cup up which can add further injury and may be what actually caused Usyk so much pain and discomfort as Usyk's reaction was more like he'd been hit in the balls.
If you watch the fight again you will see there are a substantial number of punches that Dubois threw in the same area on the belt
after that punch yet Usyk did not fall to the floor in agony nor complain about it being a low blow.
It is possible Dubois caught Usyk with a fair punch that he did not expect therefore he went down in agony, I am not going to dismiss that possibility. Usyk said it was low, the referee said it was and that is what matters. I think Dubois deserves another opportunity in the ring with Usyk but for that to happen he must defeat Joshua in September and then Usyk must defeat Fury. The results of both fights are going to be very close to call (though for many Usyk and Joshua will win their respective bouts).