If you are creating an image from a book that inspired you (not posting illustrations from the book nor portions of the book), I personally think that would be fine. You'll find throughout the galleries and history, graphics that have been inspired by literary works (the Bible being a notable example, as are graphics based on Shakespearean plays or Dickens's novels). Heinlein has been the inspiration for a great many science fiction graphics and you will often see his novels references as such. As to your other question, if the graphic artist has agreed to cooperatively work with the writer, there is no problem. The only time you'd get into trouble is if you took a graphic from the gallery and then posted it with your words without the artist giving permission or without the artist having knowledge of this use of their artwork. Others may have different viewpoints. That's mine for this evening. :)