Sunshine Photo Cart 3.4.7 was released today with a couple bug fixes and, more importantly, one of the most requested features: ability to watermark on a per-image basis!
When uploading images to a gallery and you have a watermark configured, you will see a new toggle option to enable/disable the watermarks on the images you are uploading via the browser. Watermarks will be applied based on the toggle at the moment of upload.
Sunshine Photo Cart also allows you to regenerate images for galleries. The regeneration process will obey the watermark settings for an image at the original time of upload.
You can also edit individual images and change their watermark status which will be used the next time a regeneration process is run.