Ah, good question. In the poem "In Goya's greatest scenes we seem to see" Ferlinghetti makes one overarching comparison, or parallel/analogy. The poem is divided into two sections. The first describes what a viewer sees in Goya’s paintings. From the content of this first section, Ferlinghetti sees Goya’s best work in his haunting, supernatural, and allegorical works, rather than his political works, like “Hobgoblin.” In those paintings, people are tortured, both emotionally and literally, by forces beyond them.
The second section of the poem completes the comparison, saying that we see the same thing in modern America. Now, though, the forces that are larger than people, that tortured them and loom over them, are the tools of capitalism and industrialization, like billboards. Just as the supernatural creatures eat people in Goya's paintings, so ads eat us.