How to add dynamic watermarks to images in Craft/Twig
You'd definitely need a plugin for this. Craft's native abilities don't cover watermarks.

I've never attempted it myself, so these are just "best guesses"... but give the following plugins a shot:


https://github.com/fruitstudios/AviaryImageEditor/
http://michaelrog.com/craft/imagesmith


As always, you can find a full list of existing plugins here:


  https://straightupcraft.com/craft-plugins


If none of those work for you, you may need to develop your own custom plugin to handle it.