The MonkeyBread plugin allows you to watermark images.
However it requires an image (like a transparent PNG) to act as the watermark.
You could also use FMEasyCanvas to be able to edit and save images from within a filemaker webviewer. Not automatic by any means, but closer than your current process.
There may be another way but these implementations may be the easiest.
I will have to explore plugin related options, had hoped to avoid this route since my solution is server based and it's my understanding that server based plugins are often rather pricy. I will probably not get approval to spend money to save myself a few mundane steps. Thank you for your suggestions