Thank you all for your contributions - this really is an excellent forum for this sort of query.

We'd thought of 'trompe l'oeil' but in our experience the term seems to be restricted to architectural type uses (ceilings, wall paintings etc) not small book illustrations. If we're wrong, (distinctly possible!) then perhaps that's the problem solved. An example of it being used in this way would be manna.

Emanuela, I knew the Holbein (etc) trick of distortion but didn't know the word 'anamorfosi' - thank you. I'll chase that up, but it still may not be the correct word for our purpose, as in 'The Ambassadors' (for example) the skull is merely distorted - it doesn't really have two meaningful views.

We wondered about 'pictogram' - but don't seem to be getting very far with this line of thought. Oh the delights of one small gloss.