SUSIE'S REPLY: How exciting for you! Writing a book is almost like giving birth and could be considered a labor of love. I wish you every success with your adventure!
Now to get down to the nitty gritty: First and foremost: My opinion is given for guidance only; you're advised to consult a copyright lawyer on copyright issues. My information is based on US copyright law. Remember, although there are international copyright agreements, every country has its own copyright laws.
According to the information found on dictionary.com the term "plagiarism" usually applies to literary theft. Plagiarism occurs when a writer duplicates another writer's language or ideas and then calls the work his or her own. The writers' words are protected by copyright as their legal property. It is permissible to quote another writer as long as you carefully give credit to the writer and the source of your quote.
In regards to visual art there is a myth about how different an image needs to be to not be a considered a copy. Some artists believe (and/or have even been taught) that you can legally use another artist's work if you change or alter a certain percentage of the painting. Some say changing an image 10 percent is enough to make it a new image while others say a change of 20% or 30% is required. This is absolutely not true and is only a myth. A copy is a copy.
A good rule of thumb would be when comparing the two images side by side if the another artist's work is recognizable as what your image is based on then you are risking copyright infringement.
Objects or subjects such as "teacups" or the "sunset" can not be copyrighted so that they can only be painted by a single copyright owner. Another artist's work can inspire you but the concept and composition of your painting should be uniquely your own not a copy of their work.
Since you asked for my opinion, if it were my book, I'd use my own teacups, my own sketches and my own reference photos to paint the illustrations from. Let it be totally and uniquely your own without the shadow lurking over you that you might not be doing the right thing.
Just consider any copied images you've done so far practice for what's yet to come.
My personal policy is "if in doubt, don't!" Let you conscience be your guide.
Again, I wish you success with your new book! Please let me know when it's available... I like teacups!
I look forward to seeing your original watercolor illustrations.