The original novel, The Wizard of Oz was published in 1900, and so is out of copyright in the US, and probably in most if not all other countries as well. Thus neither it, nor any of the characters contained in it, are protected by copyright. The original illustrations from that book are also not under copyright. Any new images of any of the characters, based on the book, not any film, would be copyrighted only by the artist (or the artist's employer if they were works made for hire).
Wikipedia claims that the publicity photo of Roy Bolger in costume as the Scarecrow from the 1939 MGM film is in the public domain, and this seems plausible. (Note that the photo is not a still from the film itself.) Under US law in the 1930s thru 1976, publishing a work such as a photo with no copyright notice forfeited any copyright, unless the publication was unauthorized. But the question asks if use of the photo might be claimed to be an infringement of the copyright on the film. I am inclined to doubt that such a claim would be successful, but that is an issue on which it would be a good idea to obtain quality legal advice. I am not a lawyer.
Use of the character not based on the 1939 film would not be subject to a copyright claim, in my view.
Update: If it were held that the publicity photo was a trademark of the 1939 film, or of MGM, then it would be an infringement to use it in designating a product or service, or in advertising a product or service for sale. But use of it in a book describing farm or labor conditions is probably not "use in trade" and so would not get trademark protection. Again, use of the original illustrations from 1900, or new images based on the original book and not on the film would avoid this issue. Again, expert advice on what would or would not be a trademark infringement would be a good idea to avoid possible suits, because even losing suits are expensive to defend.
All this is primarily under US law, although the general principles should apply elsewhere.