I’ve noted the phrase from the “master riddle” on each painting’s page, and here’s what I mean by that.
The book’s title page contains the clue, “To solve the hidden riddle, you must use your eyes, and find the hare in every picture that may point you to the prize.” This suggests that eyes are the pointers.
When a line is drawn from the eyes through the longest finger/biggest toe on each creature in every picture in the book, the lines point to the letters on the border, which spells out words. Using this method, the first painting reveals the word “Catherine’s,” the second painting reveals the words “long finger,” and so on. The full phrase is “Catherine’s long finger over shadows earth buried yellow amulet midday points the hour in light of equinox look you.” The first letter from each word in that phrase spells “Close by Ampthill,” a geographical confirmer. See the Sir Issac Newton painting for the more information on this method.
The solution is described by Kit Williams himself in the paperback edition of Masquerade, while John Rousseau & Mike Barker’s solution is printed in full in Quest for the Golden Hare. Information about both books can be found here.