The predicate-object of a :depicts triple could be embedded on a powerpoint diagram element. We could then ask for documents that depict a resource of interest and, conversely, ask what an ambiguous blob of pink on the monitor is actually trying to depict.
foaf:depicts would be a rdfs:subPropertyOf :depicts. :depicts is more general because other things besides foaf:Images can be communicated via a visual medium.
Is :depicts an ObjectProperty or DatatypeProperty? It could be either.