I just set up Plogger for a site I made for my brother in law and I must say I like it a lot. I was looking through all the settings and such and did not see an option for watermarking images. I talked to another person here and they said it wouldn't be hard to make that happen. So I am there for asking if maybe that feature can be added in a future update.
I created a Trac ticket for your feature request (which you can do yourself in the future using your forum login/pass), so that we of the development team will keep it in our heads moving forward.
Also I have created a how to watermark you photos here : http://www.plogger.org/forum/discussion/1779/watermarking-your-fullsize-images-plogger-version-30/#Item_19
I forgot about that full post you created for watermarking. I'm definitely using that as a starting point to develop the full on watermarking for the future release. I think I can get it integrated into the Admin -> Options panel fairly easily, but it might not make the next release since there's still some other things that will take precedence. I'll let you all know how the development comes along.