The act of interpreting a story as if each object in it had an allegorical meaning is called allegoresis. Naturally, if the subject warrants it, such an interpretation can apply to pictorial as well as to verbal art. With both kinds of art, however, the surface content need not be taken allegorically, no matter how obvious its purport, since interest in and/or awareness or knowledge of the subcutaneous content may be lacking in the percipient subject.
The latter situation seems to have carried the day when it comes to the front cover of the book based on earlier blog posts, viz. The Speaking Self: Language Lore and English Usage. Of all the persons who have commented on it to the author, only one has mentioned the Rembrandt painting (Balaam’s Ass, 1626) reproduced thereon (and then only in order to warn of the danger of its being
misinterpreted). This painting has both an indexical and a symbolic function, since it points to one of the essays (2.30) in the book while explicating the title indirectly.
C. S. Peirce says somewhere that just as we talk of a body being in motion, rather than motion being in a body, so we should consider ourselves to be in meaning (semeiosis) rather than meaning to be in us. But being located in semeiosis is not the same thing as locating ourselves in relation to the semeiotic universe at any given point. Balaam’s Ass may finally have spoken in the Bible story, but her voice has yet to be heard as far as The Speaking Self is concerned.