Given that ekphrasis means, according to the Poetry Foundation, "an ekphrastic poem is a vivid description of a scene or, more commonly, a work of art", is there a term to describe the 'opposite' where a visual artist depicts a work of verbal art?
Since the Greek word έκφραση means expression (see here , the antonym would be impression.
However, that is not the kind of antonym you're looking for, it seems.
The trick lies in the expression ekphrastic poem, which, freely translated, would be a poem that expresses (a scene or a work of art).
You do not want a poem that does the opposite, but rather a painting that does a similar thing, which could be called an ekphrastic painting.
While it doesn't have the same fine resonance as "ekphrasis" but I think "illustration" as in The Illustrated Shakespeare. There are innumerable fine art paintings, drawings, etchings, and so on illustrating textual passages from Shakespeare, Dante, the Bible, Greco-Roman myths and legends, and outside of fine art there are even more innumerable examples.