The first illustration tells in its caption that the scene comes from Trajan's Column.
The second illustration is, we are told, copied from the Arch of Severus. Details below.
The leather restorer recently returned this six-volume Polybius to the library in San Francisco where I work, and today I was resurrecting the catalog record for the books and preparing their return to the Rare Book Room. The set contains over 100 of these double-page illustrations, beautifully mounted by the 18th-century binder onto stubs that permit the pictures to open flat.