Is there any way to activate watermarks on full-size image-view with thickbox? If I deactivate the thickbox option in zenpage-theme tab, then it shows me the watermark on a fullsize image view.

So, is there a way to activate full-size-watermarks AND the thickbox option?

No, I had made a test to call a watermarked image with the iFrame mode of Thickbox but that was working sometimes and sometimes not. Watermarked full images are passed through a script which thickbox can't handle.

You could try to modify the theme to use a custom sized image probably that is watermarked.