The option to scale the watermark (or select between two separate uploaded ones) for portrait and landscape images would be a nice touch. When flicking through images, the same size watermark looks (and actually is) proportionally much larger on a portrait orientated image, than same size watermark on landscape image.
Ideally, the portrait watermark would be slightly smaller (less wide primarily) so that to the eye it doesn't seem to change font size, but nevertheless looks more proportioned in comparison with the landscape images.
Actually this is more involved than first appears, because the matter is also a function of the size of the images that the watermark is being placed on. So for instance, on the larger images, the watermark tends to look much smaller on both landscape and portrait, and so the proportion between the two isn't so prominent. But on the smallest size image, the issue is much more prominent, on on the portrait image, even a watermark that looks small on the large image size - looks oversized on small portrait, imo.