Gibbon evidently didn't think much of the arcus constantini; see Chapter XIV, the section headed his reception:
The triumphal arch of Constantine still remains a melancholy proof of the decline of the arts, and a singular testimony of the meanest vanity. As it was not possible to find in the capital of the empire a sculptor who was capable of adorning that public monument, the arch of Trajan, without any respect either for his memory or for the rules of propriety, was stripped of its most elegant figures. The difference of times and persons, of actions and characters, was totally disregarded. The Parthian captives appear prostrate at the feet of a prince who never carried his arms beyond the Euphrates; and curious antiquarians can still discover the head of Trajan on the trophies of Constantine. The new ornaments which it was necessary to introduce between the vacancies of ancient sculpture are executed in the rudest and most unskilful manner.
Pictures of the Arch of Constantine as it appeared in Gibbon's day, by
Etienne Du Pérac and Giovanni Battista Piranesi, and some modern photos. Click on thumbnails to enlarge images.
This page has plans showing what comes from where on the arch, a brief discussion of why Constantine re-cycled parts of other arches, and lots of photos. Ignore the last page of links. Many of them are broken.
LacusCurtius has The Arch of Constantine: A Piece of Quick Propaganda with false colors added to the photos "to show just how much of this edifice was patched together from the work of previous centuries"; and a page of links about the arch, with Bill Thayer's ratings of them.
