I sold an isolated vertical image today and looked at the item details page. I was amazed at how huge the image was:
Compare the size of a horizontal image on its detail page:
And here’s a more typical vertical image (native 5D Mk II image size)
It appears that your sizing algorithm picks the width and then makes the other dimension proportional. You probably need to have a maximum height, adjusting to a smaller width if appropriate. Given the size of the garland image and the relatively weak watermark, in addition to not looking very polished, it’s virtually giving away a huge size image.