One idea I have for illustrations is if the story takes place in or there are characters from another time period, I look up illustrations from that period and mimic it's style. Like say 1966 just type "1966 illustration" in Google or Bing and presto! or find books published that year and my favorites are the Illustration Society, Graphis, International Yearbook of Communication Design, etc. If you want to do any earlier time period or like say a certain person such as Cleopatra. See how other artists depicted her throughout the ages like here [link]
plus there's debates about her heritage. Let's say your story is set in Norway in the late 19th/early 20th century, which artist is most identified with this place and time? Edvard Munch! Look up some of his work [link]
and see how he painted things.
I hope that's good advice for you.