If I'm understanding this correctly- here's a couple more options that I didn't see mentioned yet.
Seems like a great use of the compound clip feature.
Put your images in the compound clip and use it on anything.
if at any time you need to tweak the watermark, or change it, just change the contents of the one compound clip and and any project that has that compound clip used will be updated (IF thats what you want to do)
The Motion template is an excellent idea, however I've run into issues when apple has updated Motion and the text engine, it broke several titles that I needed to rebuild and reinstall fo a bunch of machines, and cause a lot of headache.
Another option is using compressor, which still has the watermark feature in the very bottom of the Video tab on the right side. I typically use compressor myself as I use to send copies to different places and needed different sizes etc and compressor made things easier for me.