Is the document still in copyright? Look at the introductory section on duration, above. If copyright has expired you may reproduce the image in any way you like (see 1.4). You may make and keep a copy of any kind of copyright work for purposes of non-commercial research or private study, so long as your copying is fair dealing. For more on that see 1.1 to 1.3.
You may publish (in any way, including online) an image of a document for purposes of criticism or review, so long as it is fair dealing and there is genuine commentary on the document or on the category of works that it represents. You may also publish an image of any kind of document except a photograph for purposes of current news reporting, so long as it is fair dealing and the news is genuinely current. See 1.1 to 1.3.
If the document is a literary, dramatic or musical work (including any illustrations) that is at least 100 years old and whose author has been dead for at least 50 years, and it has never been published, you may publish it (including by reproducing it online) so long as you do not know who the present owner of the copyright is. For use on a secure network such as Weblearn or Canvas, see 4.6.
You need permission for any other use of a copyright work before you may lawfully reproduce it in a publication or online. For information on the use of orphan works (where the copyright owner is unknown or untraceable even after a diligent search) see 6.1 to 6.7. For advice on tracing rights owners see 1.10.