Well in my opinion authors/writers should be very deep thinking people.
And being a deep thinking person means thinking about emotion, life and how it works, light and dark, saintly or corrupt.
In conclusion, dark and light imagery is probably just what comes to mind when writing. To some authors/writers they welcome light illustrations into their numerous stories and books for two reasons: they want their readers to have sanctimonious happy memories of the book they read by that author. Also they dislike dark thoughts.
And for the authors/writer who choose to illustrate their books with darker imagery simply means that they are more in touch with their dark sides.