For the relevant details of any book, Sophie, click 'Search the Integrated Catalogue' on the British Library website. By law, a copy of every book published in the UK has to be deposited with the British Library. The details in the BL catalogue mirror those required by the Harvard referencing system:
http://catalogue.bl.u...le_name=login-bl-list
If your reference is specifically to a picture (rather than to written content) then, unless you're sure of the name of the illustrator, all you can do is to put something like:
Cover illustration (anon), The Answerbank for Beginners, Crystal Guides, London, 2011
If you know the name of the illustrator, something like this should do:
M. Mouse (illus.), The Answerbank for Beginners, Crystal Guides, London, 2011
As long as you keep as close as is practically possible to the Harvard referencing system (and your sources are clear), no assessor is going to penalise you for any minor discrepancies in your methodology. (I can guarantee that the guy who assessed my degree thesis never bothered checking one of my key sources. There was only one copy of the book in the UK, hundreds of miles from Sheffield university, and it was written in 18th century German - which I'd had to translate myself!)
Chris