I think the original background, so when you squared it, it added to white vertical rectangles on either side.That's kind of weird, the file i uploaded was a square background. I'll take a look sometime in a dat or two but can't be sure when.
In the meantime see if GIMP is available for Mac, it's a linux port, quite powerful, though not as powerful as Photoshop though I've never needed anything it won't do. And it's free.
Just checked, it is available for MacOS.
GIMP - Downloads
I'll look into GIMP. Thank you.