If by some reason you need to use watermark in CKFinder, then I am sure, you already knows about imagewatermark plugin which is almost in core in new CKFinder.
I want to present you something similar, but with UI.
CKFinder Watermark Plugin or just Watermark, it’s absolutely free and open-source.
In my case, client ask me for a solution that the position and image for watermark should be changed from image to image (you can imagine such case, for example if the image has dark part in bottom right). So I started to think for solutions, first variant was just with 2 select boxes, one for image and one for position, but this seems to be too flexible, I prefere just to create presets.
Anyway, I recommend just to take a look at configuration and github plugin’s home, so you will get how it works and decide if it will works for you too.