I'd reload it with the seams going horizonal,too--I've found that it saves a lot of headache! When there is a little bit of sag on the sides I will stuff a little batting between the backing and the belly bar--but you do have to be aware of it when you roll the quilt and check the underneath of the backing to make sure you don't get a tuck--so easier to just reload it.
Also--judging by the way the quilt on the bars is uneven, it doesn't look like it's a square backing.