Each panel "square" gives you a lot of wiggle room that won't be perceived despite your OCD tendencies. To prove my point, when you bought it you thought they were squares. I would pick a center size that captures the center square and the first accent border. The solid colored outer borders ( the orange around the witch, the black around the bats, the green around the cats, etc) is where you can square them up. If the smallest size side of all of them is 6 3/4" I would cut them all at 6 1/2" square, ensuring when I cut the square the picture was centered.
Toverly also offers an excellent suggestion and you can even make them wonky stars (different size points) to further distract that they are not square, which adds to the whimsy of Halloween.