It depends on the watermark what interpretation a reviewer may give (we are talking unpaid volunteers, not copyright, publishing experts, chances are they are mainly reviewing to stop trolling, offensive stuff, maybe fact checking, but complexities of copyright well beyond their (no) pay grade)
If someone sees a "(C) AN Other" type of watermark, they may well assume that is doing the job of showing copyright and that's great, they will have zero clue as to whether image is freely distributable for non profit use as long as watermark there, or whether it was added as a "get off my photos" type of thing.
Image rights / fees etc are a thing best left to the experts..
SO is currently part of a small group of academics working on a book, to make things simple, as many images as possible are those SO & other team members have produced.
However a big chunk of the upfront funding for the book is earmarked for images that will be included that are produced by other people and a specialist company is dealing with sorting out all the rights & fees. Rights specialists were mandated as part of teh whole book deal as publishers had experience of authors doing DIY rights handling and getting it badly wrong so publishers want legal liabilities covered, so experts required (as liabilities for getting it right falls on teh rights specialist company)