These verses set up multiple sets of doubled ideas. It is a poetic form that is used for emphasis. Thus, the cedars of Lebanon are paired with the oaks of Bashan. The high mountains are paired with the hills. The high towers and fenced walls visually define important cities, as do the ships of the sea and ships of Tarshish.
The message is clarified and underscored in verse 17, where the loftiness of man shall be bowed down, and the haughtiness of men shall be made low. In all of the images, something that appears tall will be brought down low. The images from the physical world are to show Jehovah’s dominance over the world, and his supremacy over even the mightiest of earth’s or man’s creations.
The real import, however, is that Jehovah judges humankind. Regardless of human accomplishments, they are for naught if one does not live according to Jehovah’s law.
Verse 16 speaks of the ships of Tarshish. The Book of Mormon adds “all the ships of the sea.” There is no indication that this is a restoration of an earlier reading. The parallelism suggests that there should be two images, and the “pleasant pictures” are certainly confusing. One scholar translates the “pleasant pictures” to mean “splendid vessels,” which fits the intended parallelism.