I you are interested in hidden watermarks, an option is as discussed here - Flickr: Discussing Fourier watermark: is it enough? in G'MIC using a different G’MIC filter. As the article says, it isn’t robust as just saving a jpeg of an original png file, re-scaling, etc will loose it. But as it is hidden, no one is going to try to remove something they don’t know is there.
It is also discussed in this forum here - Add invisible watermark to pictures
Souphead.