I think a really great way to get them to think about visual rhetoric is to get them to “adapt” an assignment into visuals.
Visual rhetoric “emphasizes images as sensory expressions of cultural meaning” Wikipedia
Contextual art analysis (dominant methodology) emphasizes aesthetics for its rhetorical functions & reception.
I’ve done exercise where students try to guess price of a book by looking at & feeling it — one blindfolded.
Visual rhetoric’s a great way to get Ss to think about argument and interpretation; multiple voices and subjectivity.
A lot of times, the visuals are a framing devise to text. How do the visuals set expectations? 
Jib Fowles’ 15 Appeals
visual rhetoric of anything is worth studying, whether it claims to be art or not.